Description of problem:
Attempting to hot add ninth vf to  guest fails, returning a generic error
message "device 'pci-assign' could not be initialized".in fact,for now.the number of vfs/pfs can be assigned to very guest is limited to 8.further more,
(qemu) console can do it like the following messages.
(qemu) device_add  pci-assign,host=09:11.0,id=vf9
pci-assign: Maximum supported assigned devices (8) already attached

Device 'pci-assign' could not be initialized


Version-Release number of selected component (if applicable):
(1)Qemu version
qemu-kvm-0.12.1.2-2.147.el6.x86_64
(2)Host kernel
#uname -r
2.6.32-117.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1.generate vfs
#modprobe -r igb
#modprobe igb max_vfs=7
2.Unbind 14 vfs form host
3.Boot guest
#/usr/libexec/qemu-kvm -m 2G -smp 4 -drive file=/root/zhangjunyi/rhel6.1-ide.qcow2,if=none,id=test,boot=on,cache=none,format=qcow2,werror=stop,rerror=stop -device virtio-blk-pci,drive=test -cpu qemu64,+sse2,+x2apic -monitor stdio -boot c -netdev tap,id=hostnet0,vhost=on -device virtio-net-pci,netdev=hostnet0,id=net0,mac=22:11:22:45:66:95 -vnc :9 -qmp tcp:0:4446,server,nowait
4.Telnet QMP
telnet 10.66.82.225 4446
Escape character is '^]'.
{"QMP": {"version": {"qemu": {"micro": 1, "minor": 12, "major": 0}, "package": "(qemu-kvm-0.12.1.2)"}, "capabilities": []}}
{"execute":"qmp_capabilities"}
{"return": {}}
5.hot added 9vfs
#{"execute":"device_add","arguments":{"driver":"pci-assign","host":"09:10.0","id":"vf1"}}
{"return": {}}
-------
until ninth vf
#{"execute":"device_add","arguments":{"driver":"pci-assign","host":"09:11.0","id":"vf9"}}

  
Actual results:
{"error": {"class": "DeviceInitFailed", "desc": "Device 'pci-assign' could not be initialized", "data": {"device": "pci-assign"}}}


Expected results:
Please refer to (qemu)'s emit messages
(qemu) device_add  pci-assign,host=09:11.0,id=vf9
pci-assign: Maximum supported assigned devices (8) already attached

Device 'pci-assign' could not be initialized


Additional info: