Description of problem:
  Failed to delete virtio-blk device by "device_del"

Version-Release number of selected component (if applicable):
  kernel version: 3.10.0-826.el7.x86_64
  qemu-kvm-rhev version: qemu-kvm-rhev-2.10.0-16.el7.x86_64

How reproducible:
  98%

Steps to Reproduce:
1.Start guest with virtio-blk data disk, cmds as bellow:
  /usr/libexec/qemu-kvm \
    -sandbox off  \
    -machine pc  \
    -nodefaults  \
    -vga cirrus  \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/var/tmp/monitor-qmpmonitor1-20170614-233639-etu9X2zc,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control  \
    -chardev socket,id=qmp_id_catch_monitor,path=/var/tmp/monitor-catch_monitor-20170614-233639-etu9X2zc,server,nowait \
    -mon chardev=qmp_id_catch_monitor,mode=control \
    -device pvpanic,ioport=0x505,id=idhq2DAN  \
    -chardev socket,id=serial_id_serial0,path=/var/tmp/serial-serial0-20170614-233639-etu9X2zc,server,nowait \
    -device isa-serial,chardev=serial_id_serial0  \
    -chardev socket,id=seabioslog_id_20170614-233639-etu9X2zc,path=/var/tmp/seabios-20170614-233639-etu9X2zc,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20170614-233639-etu9X2zc,iobase=0x402 \
    -device ich9-usb-ehci1,id=usb1,addr=0x1f.7,multifunction=on,bus=pci.0 \
    -device ich9-usb-uhci1,id=usb1.0,multifunction=on,masterbus=usb1.0,addr=0x1f.0,firstport=0,bus=pci.0 \
    -device ich9-usb-uhci2,id=usb1.1,multifunction=on,masterbus=usb1.0,addr=0x1f.2,firstport=2,bus=pci.0 \
    -device ich9-usb-uhci3,id=usb1.2,multifunction=on,masterbus=usb1.0,addr=0x1f.4,firstport=4,bus=pci.0 \
    -drive id=drive_image1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/win10-64-virtio.qcow2 \
    -device virtio-blk-pci,id=image1,drive=drive_image1,bootindex=1,bus=pci.0,addr=0x3 \
    -drive id=drive_data1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/aliang.img \
    -device virtio-blk-pci,id=data1,drive=drive_data1 \
    -device virtio-net-pci,mac=9a:43:44:45:46:47,id=idvMp6XX,vectors=4,netdev=id9qJxPT,bus=pci.0 \
    -netdev tap,id=id9qJxPT,vhost=on \
    -m 4096  \
    -smp 6,cores=2,threads=1,sockets=3  \
    -cpu host,+kvm_pv_unhalt,hv_relaxed,hv_spinlocks=0x1fff,hv_vapic,hv_time \
    -device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1  \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot order=cdn,once=d,menu=off,strict=off  \
    -no-shutdown \
    -enable-kvm \
    -monitor stdio \
    -spice ipv4,port=5000,disable-ticketing \

2. Check disk info:
   (qemu) info block
drive_image1 (#block145): /home/kvm_autotest_root/images/win10-64-virtio.qcow2 (qcow2)
    Attached to:      /machine/peripheral/image1/virtio-backend
    Cache mode:       writeback, direct

drive_data1 (#block314): /home/aliang.img (qcow2)
    Attached to:      /machine/peripheral/data1/virtio-backend
    Cache mode:       writeback, direct

        

3. Delete data disk by qmp cmd:
    {"execute":"device_del","arguments":{"id":"data1"}}

4. Wait one minute, then check disk info
    (qemu) info block
drive_image1 (#block145): /home/kvm_autotest_root/images/win10-64-virtio.qcow2 (qcow2)
    Attached to:      /machine/peripheral/image1/virtio-backend
    Cache mode:       writeback, direct

drive_data1 (#block314): /home/aliang.img (qcow2)
    Attached to:      /machine/peripheral/data1/virtio-backend
    Cache mode:       writeback, direct 


Actual results:
   No "DEVICE_DELETE" event sent out, and data disk is not unplugged.

Expected results:
  Data disk can be plugged successfully.

Additional info:
  When loading data disk as scsi-hd, it can be unplugged successfully.