Description of problem:
  Block format 'nbd' does not support reopening files when do block commit

Version-Release number of selected component (if applicable):
  kernel version: 3.10.0-1044.el7.x86_64
  qemu-kvm-rhev version: qemu-kvm-rhev-2.12.0-29.el7.x86_64

How reproducible:
 100%

Steps to Reproduce:
1.(host1)Expose image on NBD Server:
   qemu-nbd -f raw /home/kvm_autotest_root/images/rhel77-64-virtio.qcow2 -t -p 9000

2.(host2)Start guest with qemu cmds:
    /usr/libexec/qemu-kvm  \
    -name 'avocado-vt-vm1' \
    -machine pc  \
    -nodefaults \
    -device VGA,bus=pci.0,addr=0x2  \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/var/tmp/monitor-qmpmonitor1-20190123-032240-rOoB4cgD,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control  \
    -chardev socket,id=qmp_id_catch_monitor,path=/var/tmp/monitor-catch_monitor-20190123-032240-rOoB4cgD,server,nowait \
    -mon chardev=qmp_id_catch_monitor,mode=control \
    -device pvpanic,ioport=0x505,id=id8Ec4Bn  \
    -chardev socket,id=serial_id_serial0,path=/var/tmp/serial-serial0-20190123-032240-rOoB4cgD,server,nowait \
    -device isa-serial,chardev=serial_id_serial0  \
    -chardev socket,id=seabioslog_id_20190123-032240-rOoB4cgD,path=/var/tmp/seabios-20190123-032240-rOoB4cgD,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20190123-032240-rOoB4cgD,iobase=0x402 \
    -device ich9-usb-ehci1,id=usb1,addr=0x1d.7,multifunction=on,bus=pci.0 \
    -device ich9-usb-uhci1,id=usb1.0,multifunction=on,masterbus=usb1.0,addr=0x1d.0,firstport=0,bus=pci.0 \
    -device ich9-usb-uhci2,id=usb1.1,multifunction=on,masterbus=usb1.0,addr=0x1d.2,firstport=2,bus=pci.0 \
    -device ich9-usb-uhci3,id=usb1.2,multifunction=on,masterbus=usb1.0,addr=0x1d.4,firstport=4,bus=pci.0 \
    -object iothread,id=iothread0 \
    -drive id=drive_image1,if=none,snapshot=off,aio=threads,cache=none,format=qcow2,file=nbd://10.66.144.83:9000 \
    -device virtio-blk-pci,id=image1,drive=drive_image1,bus=pci.0,iothread=iothread0 \
    -drive id=drive_data1,if=none,snapshot=off,aio=threads,cache=none,format=qcow2,file=/root/test.qcow2 \
    -device virtio-blk-pci,id=data1,drive=drive_data1,bus=pci.0,iothread=iothread0 \
    -device virtio-net-pci,mac=9a:39:3a:3b:3c:3d,id=id1JNQsL,vectors=4,netdev=idVpZZ6A,bus=pci.0  \
    -netdev tap,id=idVpZZ6A,vhost=on \
    -m 8192  \
    -smp 2,maxcpus=2,cores=1,threads=1,sockets=2  \
    -cpu 'Penryn',+kvm_pv_unhalt \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1  \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=c,menu=off,strict=off \
    -enable-kvm \
    -monitor stdio \
    -qmp tcp:0:3000,server,nowait \

3.(host2)Create 4 snapshots:
  { "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1", "snapshot-file": "/root/sn1", "format": "qcow2" } }
  { "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1", "snapshot-file": "/root/sn2", "format": "qcow2" } }
  { "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1", "snapshot-file": "/root/sn3", "format": "qcow2" } }
  { "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1", "snapshot-file": "/root/sn4", "format": "qcow2" } }

4.In guest, write some data to disk
  (guest)# dd if=/dev/urandom of=test bs=1M count=600

5.(host2)Do block commit from sn4 to base
   { "execute": "block-commit", "arguments": { "device": "drive_image1","base": "nbd://10.66.144.83:9000","top":"/root/sn4"}}
{"error": {"class": "GenericError", "desc": "Block format 'nbd' used by node '#block093' does not support reopening files"}}

Actual results:
 Block commit in step5 failed, for nbd don't support reopening files.


Expected results:
 Block commit in step5 can executed successfully.