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Description of problem:
after guest boot. use 'top' and 'cat /proc/cpuinfo' cann't get vcpu but 'ls
/sys/device/system/cpus/' can see added vcpu in guest

Version-Release number of selected component (if applicable):
host kernel:
# uname -r
2.6.32-257.el6.x86_64

qemu vesion:
qemu-kvm-0.12.1.2-2.265.el6.x86_64

How reproducible:
100%

Steps to Reproduce:
1./usr/libexec/qemu-kvm -M rhel6.3.0 -cpu host --enable-kvm -m 4G -smp
1,maxcpus=161 -name rhel6.3 -uuid ddcbfb49-3411-1701-3c36-6bdbc00bedbb -rtc
base=utc,clock=host,driftfix=slew -drive
file=/dev/vg-54/lvtest,if=none,id=ide,format=qcow2,cache=none,werror=stop,rerror=stop
-device ide-drive,drive=ide,id=drive-ide0-0-0,bootindex=1 -netdev
tap,id=hostnet1 -device e1000,netdev=hostnet1,id=net1,mac=86:12:50:a4:32:72
-spice port=5911,disable-ticketing -vga qxl -device sga -chardev
socket,id=serial0,path=/var/test1,server,nowait -device
isa-serial,chardev=serial0 -balloon virtio -monitor
unix:/tmp/monitor2,server,nowait -monitor stdio

2.hotplug vcpus during booting. 
cpu_set 1 online
cpu_set 2 online
3.
  
Actual results:
hotplug fail

Expected results:
hotplug successfully and guest work well

Additional info:

cpus are onlined by udev, so you have to have a fully booted guest for hot-plugged cpu to be onlined and visible in /proc/cpuinfo