Description of problem:
Xen now support cpu hotplug for less cpu count, but not support hotplug for more cpu count.

Version-Release number of selected component (if applicable):
RHEL5.6-Server-x86_64-xen
kernel 2.6.18-227.el5xen 
libvirt-0.8.2-6.el5
kvm-83-205.el5
kvm-qemu-img-83-205.el5

How reproducible:
always

Steps to Reproduce:
1.virsh dumpxml demo
<domain type='xen' id='56'>
  <name>demo</name>
  <uuid>6df7a931-7233-bdb1-a3f1-b04793816fe8</uuid>
  <memory>2097152</memory>
  <currentMemory>2097152</currentMemory>
  <vcpu>1</vcpu>
  <bootloader>/usr/bin/pygrub</bootloader>
  <os>
    <type>linux</type>
    <kernel>/var/lib/xen/boot_kernel.8Yd4yA</kernel>
    <initrd>/var/lib/xen/boot_ramdisk.hqsZCz</initrd>
    <cmdline>ro root=/dev/VolGroup00/LogVol00 rhgb quiet</cmdline>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <disk type='file' device='disk'>
      <driver name='tap' type='aio'/>
      <source file='/var/lib/xen/images/demo.img'/>
      <target dev='xvda' bus='xen'/>
    </disk>
    <interface type='bridge'>
      <mac address='00:16:36:75:cb:96'/>
      <source bridge='virbr0'/>
      <script path='vif-bridge'/>
      <target dev='vif56.0'/>
    </interface>
    <console type='pty' tty='/dev/pts/14'>
      <source path='/dev/pts/14'/>
      <target port='0'/>
    </console>
    <input type='mouse' bus='xen'/>
    <graphics type='vnc' port='5906' autoport='yes' keymap='en-us'/>
  </devices>
</domain>
virsh start demo

2.# virsh vcpuinfo demo
VCPU:           0
CPU:            2
State:          idle
CPU time:       249.2s
CPU Affinity:   yyyy

3.# virsh setvcpus demo 2
error: Too many virtual CPUs.


Actual results:
error occured

Expected results:
hotplug should be successful

Additional info: