vCPU hot-add makes the guest abort

[Yongjie Ren]

Environment:
------------
Host OS (ia32/ia32e/IA64):ia32e
Guest OS (ia32/ia32e/IA64):ia32e
Guest OS Type (Linux/Windows):Linux(RHEL6u1)
kvm.git Commit:ae7a2a3fb6f8b784c2752863f4f1f20c656f76fb
qemu-kvm Commit:a212f79fc4596570124fb864425b980c157cd001
Host Kernel Version:3.5.0-rc1
Hardware:Romley-EP, WSM-EP

Bug detailed description:
--------------------------
Hot-add vcpu for a guest in qemu monitor will make the guest abort. (QEMU process is killed.)
After Hot-removing vcpu for the guest in qemu monitor, the guest will be still alive but the the number of vCPU in the guest doesn't decrease.

Reproduce steps:
----------------
Hot-add vCPU:
1.start up a host with kvm (commit: ae7a2a3f)and qemu-kvm(commit: a212f79f..)
2.qemu-system-x86_64 -m 1024 -smp 4,maxvcpus=10 –hda /root/rhel6u3.qcow
3. ls /sys/devices/system/cpu/ (to check CPUs in guest)
4. ctrl+alt+2 (swith to qemu monitor)
5. cpu_set 5 online

Hot-remove vCPU:
1.start up a host with kvm (commit: ae7a2a3f) and qemu-kvm(commit: a212f79f..)
2.qemu-system-x86_64 -m 1024 -smp 4,maxvcpus=10 –hda /root/rhel6u3.qcow
3. ls /sys/devices/system/cpu/ (to check CPUs in guest)
4. ctrl+alt+2 (swith to qemu monitor)
5. cpu_set 2 offline
6. ctrl+alt+1
7. ls /sys/devices/system/cpu/ (to check CPUs in guest)

Current result:
----------------
vCPU hot-add will cause guest abort.
vCPU hot-remove doesn't decrease the number of vCPU in guest.

Expected result:
----------------
The number of the guest’s cpu changes as we expect.

Basic root-causing log:
----------------------
[root@vt-snb9 ~]# qemu-system-x86_64 -m 1024 -smp 4,maxcpus=10 -hda
/root/rhel6u3.qcow
VNC server running on `::1:5900'
qemu-system-x86_64: hw/qdev.c:84: qdev_set_parent_bus: Assertion
`bus->allow_hotplug' failed.
Aborted (core dumped)

[chao zhou (chao-zhou)]

qemu-kvm.git has merged to qemu.git, upstream qemu doesn't have cpu_set command.