Description of problem:
Start a guest with virtio video device with heads >1, e.g.heads=3.  If do not select the checkbox in virt-viewer to show the 'Display 2', then qemu will crash when take screenshot for the 2nd display.
(If show it by virt-viewer, issue will not happen.)

Version-Release number of selected component (if applicable):
qemu-kvm-core-2.12.0-63.module+el8+2833+c7d6d092
libvirt-4.5.0-23.module+el8+2800+2d311f65.x86_64
kernel-4.18.0-73.el8.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Configure a guest with following xml(a rhel guest as example):
...
    <channel type='spicevmc'>
      <target type='virtio' name='com.redhat.spice.0'/>
      <address type='virtio-serial' controller='0' bus='0' port='2'/>
    </channel>
...
    <graphics type='spice' autoport='yes'>
      <listen type='address'/>
    </graphics>
...
    <video>
      <model type='virtio' heads='3' primary='yes'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x0'/>
    </video>
...
2. Start guest
# virsh start rhel8.0-yqz
Domain rhel8.0-yqz started

...-device virtio-vga,id=video0,max_outputs=3,bus=pci.0,addr=0x2 ...


3. Do not show the 2nd display by virt-viewer, try to take screenshot for each video head:
# virsh screenshot rhel8.0-yqz --screen 0
Screenshot saved to rhel8.0-yqz-2019-02-28-19:46:24.ppm, with type of image/x-portable-pixmap
# virsh screenshot rhel8.0-yqz --screen 1
 
error: could not take a screenshot of rhel8.0-yqz
error: Unable to read from monitor: Connection reset by peer

# abrt-cli ls
id 1e2225e60b90d41d0677df534053c9631e29439f
reason:         qmp_screendump(): qemu-kvm killed by SIGSEGV
time:           Thu 28 Feb 2019 07:46:25 PM CST
cmdline:        /usr/libexec/qemu-kvm -name guest=rhel8.0-yqz,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-13-rhel8.0-yqz/master-key.aes -machine pc-q35-rhel7.6.0,accel=kvm,usb=off,vmport=off,dump-guest-core=off -cpu Haswell-noTSX-IBRS,vme=on,ds=on,acpi=on,ss=on,ht=on,tm=on,pbe=on,dtes64=on,monitor=on,ds_cpl=on,vmx=on,smx=on,est=on,tm2=on,xtpr=on,pdcm=on,osxsave=on,f16c=on,rdrand=on,arat=on,tsc_adjust=on,stibp=on,xsaveopt=on,pdpe1gb=on,abm=on -m 1024 -realtime mlock=off -smp 1,sockets=1,cores=1,threads=1 -uuid 74e8cfa0-0beb-40ac-9662-17d5da05c52d -no-user-config -nodefaults -chardev socket,id=charmonitor,fd=30,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc,driftfix=slew -global kvm-pit.lost_tick_policy=delay -no-hpet -no-shutdown -global ICH9-LPC.disable_s3=1 -global ICH9-LPC.disable_s4=1 -boot strict=on -device pcie-root-port,port=0x10,chassis=1,id=pci.1,bus=pcie.0,multifunction=on,addr=0x2 -device pcie-root-port,port=0x11,chassis=2,id=pci.2,bus=pcie.0,addr=0x2.0x1 -device pcie-root-port,port=0x12,chassis=3,id=pci.3,bus=pcie.0,addr=0x2.0x2 -device pcie-root-port,port=0x13,chassis=4,id=pci.4,bus=pcie.0,addr=0x2.0x3 -device pcie-root-port,port=0x14,chassis=5,id=pci.5,bus=pcie.0,addr=0x2.0x4 -device pcie-root-port,port=0x15,chassis=6,id=pci.6,bus=pcie.0,addr=0x2.0x5 -device pcie-root-port,port=0x16,chassis=7,id=pci.7,bus=pcie.0,addr=0x2.0x6 -device qemu-xhci,p2=15,p3=15,id=usb,bus=pci.2,addr=0x0 -device virtio-serial-pci,id=virtio-serial0,bus=pci.3,addr=0x0 -drive file=/s3-qe-team/yanqzhan/RHEL-8.0-x86_64-latest.qcow2,format=qcow2,if=none,id=drive-virtio-disk0 -device virtio-blk-pci,scsi=off,bus=pci.4,addr=0x0,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -netdev tap,fd=32,id=hostnet0,vhost=on,vhostfd=33 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:3a:3d:9f,bus=pci.1,addr=0x0 -chardev pty,id=charserial0 -device isa-serial,chardev=charserial0,id=serial0 -chardev socket,id=charchannel0,fd=34,server,nowait -device virtserialport,bus=virtio-serial0.0,nr=1,chardev=charchannel0,id=channel0,name=org.qemu.guest_agent.0 -chardev spicevmc,id=charchannel1,name=vdagent -device virtserialport,bus=virtio-serial0.0,nr=2,chardev=charchannel1,id=channel1,name=com.redhat.spice.0 -device usb-tablet,id=input0,bus=usb.0,port=1 -spice port=5900,addr=127.0.0.1,disable-ticketing,seamless-migration=on -device virtio-vga,id=video0,max_outputs=2,bus=pcie.0,addr=0x1 -device ich9-intel-hda,id=sound0,bus=pcie.0,addr=0x1b -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -chardev spicevmc,id=charredir0,name=usbredir -device usb-redir,chardev=charredir0,id=redir0,bus=usb.0,port=2 -chardev spicevmc,id=charredir1,name=usbredir -device usb-redir,chardev=charredir1,id=redir1,bus=usb.0,port=3 -device virtio-balloon-pci,id=balloon0,bus=pci.5,addr=0x0 -object rng-random,id=objrng0,filename=/dev/urandom -device virtio-rng-pci,rng=objrng0,id=rng0,bus=pci.6,addr=0x0 -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg timestamp=on
package:        15:qemu-kvm-core-2.12.0-63.module+el8+2833+c7d6d092
uid:            107 (qemu)
count:          1
Directory:      /var/spool/abrt/ccpp-2019-02-28-19:46:25-3153
Run 'abrt-cli report /var/spool/abrt/ccpp-2019-02-28-19:46:25-3153' for creating a case in Red Hat Customer Portal


Actual result:
As in step3, qemu crashed when take screenshot for 2nd head of virtio video device when the display not shows in virt-viewer.

Expected result:
qemu should not crash, screenshot could succeed or fail with proper error.

Additional info:
1.(gdb) bt
#0  0x0000559c66413595 in ppm_save (errp=0x7ffdff574a58, ds=0x0, filename=0x559c69054490 "/var/cache/libvirt/qemu/qemu.screendump.89EaXo") at ui/console.c:373
#1  0x0000559c66413595 in qmp_screendump
    (filename=0x559c69054490 "/var/cache/libvirt/qemu/qemu.screendump.89EaXo", has_device=<optimized out>, device=<optimized out>, has_head=<optimized out>, head=<optimized out>, errp=errp@entry=0x7ffdff574a58)
    at ui/console.c:373
#2  0x0000559c663017a7 in qmp_marshal_screendump (args=<optimized out>, ret=<optimized out>, errp=0x7ffdff574ac8) at qapi/qapi-commands-ui.c:110
#3  0x0000559c664e93bb in do_qmp_dispatch (errp=0x7ffdff574ac0, request=0x7ffdff574ac0, cmds=<optimized out>) at qapi/qmp-dispatch.c:111
#4  0x0000559c664e93bb in qmp_dispatch (cmds=<optimized out>, request=request@entry=0x559c67de0e80) at qapi/qmp-dispatch.c:160
#5  0x0000559c6620b62e in monitor_qmp_dispatch_one (req_obj=<optimized out>) at /usr/src/debug/qemu-kvm-2.12.0-63.module+el8+2833+c7d6d092.x86_64/monitor.c:4084
#6  0x0000559c6620b8cc in monitor_qmp_bh_dispatcher (data=<optimized out>) at /usr/src/debug/qemu-kvm-2.12.0-63.module+el8+2833+c7d6d092.x86_64/monitor.c:4142
#7  0x0000559c664f3bf6 in aio_bh_call (bh=0x559c67c0e950) at util/async.c:118
#8  0x0000559c664f3bf6 in aio_bh_poll (ctx=ctx@entry=0x559c67bc7d70) at util/async.c:118
#9  0x0000559c664f6d74 in aio_dispatch (ctx=0x559c67bc7d70) at util/aio-posix.c:440
#10 0x0000559c664f3ad2 in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at util/async.c:261
#11 0x00007f4f0ebb989d in g_main_dispatch (context=0x559c67bc8540) at gmain.c:3176
#12 0x00007f4f0ebb989d in g_main_context_dispatch (context=context@entry=0x559c67bc8540) at gmain.c:3829
#13 0x0000559c664f5ff0 in glib_pollfds_poll () at util/main-loop.c:215
#14 0x0000559c664f5ff0 in os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:238
#15 0x0000559c664f5ff0 in main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:497
#16 0x0000559c661c04b5 in main_loop () at vl.c:1964
#17 0x0000559c661c04b5 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4789

2. If select to show 'Display 2' in virt-viewer, screenshot succeed:
# virsh screenshot avocado-vt-vm1 --screen 1
Screenshot saved to avocado-vt-vm1-2019-02-28-07:20:57.ppm, with type of image/x-portable-pixmap