Description of problem:
Core dump when do 'block-job-complete' after 'drive-mirror'.

Version-Release number of selected component (if applicable):
Host:
  kernel-3.10.0-408.el7.x86_64
  qemu-kvm-rhev-2.6.0-3.el7.x86_64
Guest:
  kernel-3.10.0-408.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Start a VM using following commands:
 ...
 -object iothread,id=iothread0 \
 -drive file=sys.qcow2,format=qcow2,if=none,cache=none,id=drive_sysdisk,werror=stop,rerror=stop \
 -device virtio-blk-pci,drive=drive_sysdisk,id=device_sysdisk,iothread=iothread0,bootindex=0 \
 -drive file=datadisk.qcow2,format=qcow2,if=none,cache=none,id=drive_datadisk1,werror=stop,rerror=stop \
 -device virtio-blk-pci,drive=drive_datadisk1,id=device_datadisk1,iothread=iothread0 \
 ...

2.Block mirror
{"execute":"drive-mirror","arguments":{"device":"drive_datadisk1","target":"dest_datadisk1.qcow2","format":"qcow2","mode":"absolute-paths","sync":"full","speed":1000000000,"on-source-error": "stop", "on-target-error": "stop" } }
{"return": {}}

{"timestamp": {"seconds": 1464157486, "microseconds": 965574}, "event": "BLOCK_JOB_READY", "data": {"device": "drive_datadisk1", "len": 536870912, "offset": 536870912, "speed": 1000000000, "type": "mirror"}}

(qemu) info block-jobs 
Type mirror, device drive_datadisk1: Completed 536870912 of 536870912 bytes, speed limit 1000000000 bytes/s

3.Block job complete
{ "execute": "block-job-complete", "arguments": { "device": "drive_datadisk1"}}
{"return": {}}
{"timestamp": {"seconds": 1464157596, "microseconds": 264218}, "event": "BLOCK_JOB_COMPLETED", "data": {"device": "drive_datadisk1", "len": 536870912, "offset": 536870912, "speed": 1000000000, "type": "mirror"}}


(qemu) qemu-kvm: block.c:2338: bdrv_delete: Assertion `bdrv_op_blocker_is_empty(bs)' failed.
0boot.sh: line 38: 26338 Aborted                 (core dumped)

Actual results:
Core dumped.

Expected results:
Block mirror should work well with data-plane.

Additional info:
1.Not reproducible with qemu-kvm-rhev-2.3.0-31.el7_2.12.x86_64.
2.Backtrace
(gdb) bt
#0  0x00007f10d44915f7 in raise () from /lib64/libc.so.6
#1  0x00007f10d4492ce8 in abort () from /lib64/libc.so.6
#2  0x00007f10d448a566 in __assert_fail_base () from /lib64/libc.so.6
#3  0x00007f10d448a612 in __assert_fail () from /lib64/libc.so.6
#4  0x00007f10dc9f856f in bdrv_delete (bs=0x7f10de16e800) at block.c:2338
#5  bdrv_unref (bs=0x7f10de16e800) at block.c:3376
#6  0x00007f10dc9fb353 in block_job_defer_to_main_loop_bh (opaque=0x7f10de038310) at blockjob.c:476
#7  0x00007f10dc9f3fcd in aio_bh_call (bh=<optimized out>) at async.c:66
#8  aio_bh_poll (ctx=ctx@entry=0x7f10de109980) at async.c:94
#9  0x00007f10dc9fdb70 in aio_dispatch (ctx=0x7f10de109980) at aio-posix.c:308
#10 0x00007f10dc9f3dbe in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, 
    user_data=<optimized out>) at async.c:233
---Type <return> to continue, or q <return> to quit---
#11 0x00007f10d528f79a in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
#12 0x00007f10dc9fc460 in glib_pollfds_poll () at main-loop.c:213
#13 os_host_main_loop_wait (timeout=<optimized out>) at main-loop.c:258
#14 main_loop_wait (nonblocking=<optimized out>) at main-loop.c:506
#15 0x00007f10dc7c655f in main_loop () at vl.c:1934
#16 main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4667