Description of problem:
fail to unhotplug virtio block disk while the device is using inside win8-64 guest. Or qemu monitor or QMP give friendly messages while failed(e.g device is busying).

Current, qemu monitor and QMP don't any output message when hot-unplug fail.

Version-Release number of selected component (if applicable):
qemu-kvm-1.5.2-4.el7.x86_64
kernel 3.10.0-11.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Boot win8-64 guest with a virtio-blk data device.
# /usr/libexec/qemu-kvm  -S -M pc-i440fx-rhel7.0.0 \
-cpu SandyBridge -enable-kvm \
-m 4G -smp 4,sockets=2,cores=2,threads=1 \
-name juli -uuid 355a2475-4e03-4cdd-bf7b-5d6a59edaa61 \
-rtc base=localtime,clock=host,driftfix=slew \
-drive file=/mnt/win8-64.raw,if=none,cache=none,aio=native,format=raw,id=drive0  \
-device virtio-blk-pci,bus=pci.0,addr=0x8,drive=drive0,scsi=off,config-wce=off,bootindex=0  \
-drive file=/mnt/sdb-1.raw,if=none,id=drive-virtio0-0-1,media=disk,cache=none,format=raw \
-device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio0-0-1,id=test30-d,scsi=off,config-wce=off \
-device virtio-balloon-pci,id=ballooning,addr=0x6 -global PIIX4_PM.disable_s3=0 -global PIIX4_PM.disable_s4=0 \
-netdev tap,id=hostnet0,vhost=off,queues=4,script=/etc/qemu-ifup \
-device virtio-net-pci,mq=on,vectors=17,netdev=hostnet0,id=virtio-net-pci0,mac=24:be:05:14:0d:52,addr=0x7,bootindex=2 \
-k en-us -boot menu=on,reboot-timeout=-1,strict=on \
-qmp tcp:0:4445,server,nowait -serial unix:/tmp/ttyS0,server,nowait \
-vnc :3 -spice port=5932,disable-ticketing \
-vga cirrus -monitor stdio \
-monitor tcp:0:7445,server,nowait \
-drive file=/mnt/virtio-win-prewhql-0.1-68/virtio-win-prewhql-0.1.iso,if=none,media=cdrom,format=raw,aio=native,id=drive-ide1-0-2 \
-device ide-drive,drive=drive-ide1-0-2,id=ide1-0-2,bus=ide.1,unit=1 

2.Do hot-unplug inside monitor and QMP.
(qemu) device_del test30-d
{"execute":"device_del","arguments":{"id":"test30-d"}}

3.Check the result.
(qemu) info qtree

Actual results:
(qemu) device_del test30-d

{"execute":"device_del","arguments":{"id":"test30-d"}}
{"return": {}}

(qemu) info qtree
... ...
 dev: virtio-blk-pci, id "test30-d"
        class = 0x0
        ioeventfd = on
        vectors = 2
        x-data-plane = off
        indirect_desc = on
        event_idx = on
        drive = drive-virtio0-0-1
        logical_block_size = 512
        physical_block_size = 512
        min_io_size = 0
        opt_io_size = 0
        bootindex = -1
... ...


Expected results:
If the device can not hot-unplug successfully, monitor or QMP will give friendly messages.
Hot-unplug can success while the device is using.

Additional info:
Hot-unplug can success while the device is not doing I/O service.

I alse Rhel7.0 guest:
With Rhel7.0 guest hot-unplug can success while the device is using.