Description of problem:
Boot guest with balloon device, but uninstall or disabled balloon driver inside guest, and then balloon the memory, there's no prompt.
For rhel guest, rmmod virtio_balloon 
For win guest, disable/uninstall balloon driver from device manager.

Version-Release number of selected component (if applicable):
kernel-2.6.32-171.el6.x86_64
qemu-kvm-0.12.1.2-2.171.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Boot a guest with balloon device
/usr/libexec/qemu-kvm -m 4G -smp 4 -cpu cpu64-rhel6,+x2apic -usbdevice tablet -drive file=win7-64-virtio.qcow2,format=qcow2,if=none,id=drive-ide0-0-0,werror=stop,rerror=stop,cache=none -device virtio-blk-pci,drive=drive-ide0-0-0,id=ide0-0-0 -netdev tap,id=hostnet0,script=/etc/qemu-ifup -device e1000,netdev=hostnet0,mac=00:10:1a:20:06:28,bus=pci.0,addr=0x6 -boot dc -uuid bea9c85e-0a47-4030-979d-5d94bb3d6161 -rtc-td-hack -no-kvm-pit-reinjection -monitor stdio -qmp tcp:0:4444,server,nowait -name win7-64-balloon -spice disable-ticketing,port=5931 -vga qxl -device virtio-balloon-pci,bus=pci.0,id=balloon0

2. Disable balloon driver inside guest.
3. Balloon memory
{"execute":"balloon","arguments":{"value":2147483648}}
{"return": {}}
  
Actual results:
No prompt telling user the balloon is disabled/uninstalled.

Expected results:
It will be fine to have a prompt.

Additional info: