Description of problem:
qemu abort when hot unplug block device during live commit.

Version-Release number of selected component (if applicable):
kernel-3.10.0-640.el7.x86_64
qemu-kvm-rhev-2.9.0-5.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Launch guest
(gdb) run -name 'avocado-vt-vm1' \
    -vga cirrus \
    -device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pci.0,addr=03 \
    -drive id=drive_image1,if=none,snapshot=off,aio=native,cache=none,format=raw,file=/home/kvm_autotest_root/images/rhel74-64-virtio-scsi.raw \
    -device scsi-hd,id=image1,drive=drive_image1 \
    -device virtio-net-pci,mac=9a:ac:ad:ae:af:b0,id=idqKTtyC,vectors=4,netdev=idBAxKKY,bus=pci.0,addr=04 \
    -netdev tap,id=idBAxKKY,vhost=on \
    -m 4096 \
    -smp 4,cores=2,threads=1,sockets=2 \
    -cpu 'SandyBridge',+kvm_pv_unhalt \
    -vnc :0 \
    -rtc base=utc,clock=host,driftfix=slew \
    -boot order=cdn,once=c,menu=off,strict=off \
    -monitor stdio \
    -qmp tcp::5555,server,nowait

2. Live snapshot:
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1","snapshot-file": "/mnt/iscsi/sn1", "format": "qcow2", "mode": "absolute-paths" } }
{"return": {}}
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1","snapshot-file": "/mnt/iscsi/sn2", "format": "qcow2", "mode": "absolute-paths" } }
{"return": {}}
{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1","snapshot-file": "/mnt/iscsi/sn3", "format": "qcow2", "mode": "absolute-paths" } }

dd a 1G file on sn3.

{ "execute": "blockdev-snapshot-sync", "arguments": { "device": "drive_image1","snapshot-file": "/mnt/iscsi/sn4", "format": "qcow2", "mode": "absolute-paths" } }

3. Live commit from sn3 to sn1:
{ "execute": "block-commit", "arguments": { "device": "drive_image1", "base": "/mnt/iscsi/sn1", "top": "/mnt/iscsi/sn3"}}

4. Hot unplug the block device:
{ "execute": "device_del", "arguments": { "id": "image1"}}


Actual results:
qemu abort:
main-loop: WARNING: I/O thread spun for 1000 iterations
[New Thread 0x7ffedf9ff700 (LWP 26964)]
qemu-kvm: block.c:1894: bdrv_attach_child: Assertion `parent_bs->drv' failed.

Program received signal SIGABRT, Aborted.
0x00007fffed4b81f7 in raise () from /lib64/libc.so.6


Expected results:
Hot unplug succeeds, block job being cancelled.

Additional info:
qemu-kvm-rhev-2.6.0-27.el7.x86_64 has no this issue, so this is a regression.