Description of problem:
start a guest with vtpm, destroy it, then try to remove vtpm from
guest xml, libvirtd will crash

Version-Release number of selected component (if applicable):
libvirt-daemon-5.6.0-1.module+el8.1.0+3890+4d3d259c.x86_64
qemu-kvm-4.0.0-6.module+el8.1.0+3736+a2aefea3.x86_64

How reproducible:
100%

Steps to Reproduce:
0. #yum install swtpm swtpm-tools

1. start a guest with vtpm xml as below:
    <tpm model='tpm-crb'>
      <backend type='emulator' version='2.0'/>
    </tpm>

2. destroy the guest

3. try to remove the vtpm device:
# virsh edit rhel8.1-ovmf
error: Disconnected from qemu:///system due to end of file
error: End of file while reading data: Input/output error
Failed. Try again? [y,n,i,f,?]:
error: internal error: client socket is closed

Or: # virsh define rhel8.1-ovmf.xml-bak 
error: Disconnected from qemu:///system due to end of file
error: Failed to define domain from rhel8.1-ovmf.xml-bak
error: End of file while reading data: Input/output error

# abrt-cli ls
id e600f7c12b67cfa0c69b37d50aa83c19768cd903
reason:         virDomainCheckDeviceChanges(): libvirtd killed by SIGSEGV
time:           Thu 08 Aug 2019 10:44:21 PM EDT
cmdline:        /usr/sbin/libvirtd --timeout 120
package:        libvirt-daemon-5.6.0-1.module+el8.1.0+3890+4d3d259c
uid:            0 (root)
count:          1
Directory:      /var/spool/abrt/ccpp-2019-08-08-22:44:21-20803
Run 'abrt-cli report /var/spool/abrt/ccpp-2019-08-08-22:44:21-20803'
for creating a case in Red Hat Customer Portal

(gdb) bt
#0  virDomainCheckTPMChanges (def=<optimized out>, newDef=0x7fd7ec0cb760) at conf/domain_conf.c:31469
#1  virDomainCheckDeviceChanges (def=<optimized out>, newDef=newDef@entry=0x7fd7ec0cb760) at conf/domain_conf.c:31493
#2  0x00007fd7d8c807fc in qemuDomainCheckDeviceChanges (def=0x7fd7ec0cb760, driver=0x7fd7b810e5d0) at qemu/qemu_driver.c:7779
#3  qemuDomainDefineXMLFlags (conn=0x7fd8080043d0, 
    xml=0x7fd7ec002bc0 "<domain type='kvm'>\n  <name>rhel8.1-ovmf</name>\n  <uuid>0572326a-499d-407e-8d4a-2a8e974c9496</uuid>\n  <metadata>\n    <libosinfo:libosinfo xmlns:libosinfo=\"http://libosinfo.org/xmlns/libvirt/domain/1.0"..., flags=<optimized out>) at qemu/qemu_driver.c:7823
#4  0x00007fd821d4cd24 in virDomainDefineXMLFlags (conn=conn@entry=0x7fd8080043d0, 
    xml=0x7fd7ec002bc0 "<domain type='kvm'>\n  <name>rhel8.1-ovmf</name>\n  <uuid>0572326a-499d-407e-8d4a-2a8e974c9496</uuid>\n  <metadata>\n    <libosinfo:libosinfo xmlns:libosinfo=\"http://libosinfo.org/xmlns/libvirt/domain/1.0"..., flags=1) at libvirt-domain.c:6197
#5  0x000055beb6ad2d06 in remoteDispatchDomainDefineXMLFlags (server=0x55beb7c21090, msg=0x55beb7c63a60, args=0x7fd7ec002980, args=0x7fd7ec002980, ret=0x7fd7ec0029e0, rerr=0x7fd80fffe960, 
    client=<optimized out>) at remote/remote_daemon_dispatch_stubs.h:5198
#6  remoteDispatchDomainDefineXMLFlagsHelper (server=0x55beb7c21090, client=<optimized out>, msg=0x55beb7c63a60, rerr=0x7fd80fffe960, args=0x7fd7ec002980, ret=0x7fd7ec0029e0)
    at remote/remote_daemon_dispatch_stubs.h:5176
#7  0x00007fd821c7e774 in virNetServerProgramDispatchCall (msg=0x55beb7c63a60, client=0x55beb7c63c10, server=0x55beb7c21090, prog=0x55beb7c5e430) at rpc/virnetserverprogram.c:435
#8  virNetServerProgramDispatch (prog=0x55beb7c5e430, server=server@entry=0x55beb7c21090, client=0x55beb7c63c10, msg=0x55beb7c63a60) at rpc/virnetserverprogram.c:302
#9  0x00007fd821c83a2c in virNetServerProcessMsg (msg=<optimized out>, prog=<optimized out>, client=<optimized out>, srv=0x55beb7c21090) at rpc/virnetserver.c:137
#10 virNetServerHandleJob (jobOpaque=<optimized out>, opaque=0x55beb7c21090) at rpc/virnetserver.c:158
#11 0x00007fd821ba2690 in virThreadPoolWorker (opaque=opaque@entry=0x55beb7c02a10) at util/virthreadpool.c:163
#12 0x00007fd821ba199c in virThreadHelper (data=<optimized out>) at util/virthread.c:206
#13 0x00007fd81f4452de in start_thread (arg=<optimized out>) at pthread_create.c:486
#14 0x00007fd81e923133 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95



Actual results:
As in step3.

Expected results:
Should remove successfully or fail with proper error, instead of crashing libvirtd.

Additional info:
Remove the storagefile will make it succeed: #rm -rf /var/lib/libvirt/swtpm/0572326a-499d-407e-8d4a-2a8e974c9496