Description of problem:
Guest name icon disappear for lxc running guest when remount cgroup filesystem.

Version-Release number of selected component (if applicable):
libvirt-0.9.4-23.el6.x86_64
qemu-kvm-0.12.1.2-2.211.el6.x86_64

# uname -a
Linux wjw.redhat.com 2.6.32-220.el6.x86_64 #1 SMP Wed Nov 9 08:03:13 EST 2011 x86_64 x86_64 x86_64 GNU/Linux

How reproducible:
100%

Steps to Reproduce:
1. Prepare a application container guest.
# virsh -c lxc:///
virsh # dumpxml demo
<domain type='lxc'>
  <name>demo</name>
  <uuid>66c3867f-9603-daac-0d1e-48707d549823</uuid>
  <memory>1048576</memory>
  <currentMemory>819200</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64'>exe</type>
    <init>/bin/sh</init>
  </os>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/libvirt_lxc</emulator>
    <interface type='bridge'>
      <mac address='00:16:3e:ec:b3:7f'/>
      <source bridge='br0'/>
      <target dev='veth0'/>
    </interface>
    <console type='pty'>
      <target type='serial' port='0'/>
    </console>
  </devices>
</domain>

2. Launch virt-manager and run the guest by virt-manager.
#virt-manager --debug

3. Check the guest status:
#virsh -c lxc:///
virsh # dominfo demo
Id:             17425
Name:           demo
UUID:           66c3867f-9603-daac-0d1e-48707d549823
OS Type:        exe
State:          running
CPU(s):         1
CPU time:       0.0s
Max memory:     1048576 kB
Used memory:    432 kB
Persistent:     yes
Autostart:      disable
Managed save:   unknown

4. Umount the cgroup filesystem:
# /etc/init.d/cgconfig stop
Stopping cgconfig service:                                 [  OK  ]
# /etc/init.d/libvirtd restart
Stopping libvirtd daemon:                                  [  OK  ]
Starting libvirtd daemon:                                  [  OK  ]
# lscgroup
cgroups can't be listed: Cgroup is not mounted

5. Check the guest again:
# virsh -c lxc:///
virsh # dominfo demo
Id:             17425
Name:           demo
UUID:           66c3867f-9603-daac-0d1e-48707d549823
OS Type:        exe
State:          running
CPU(s):         1
Max memory:     1048576 kB
Used memory:    819200 kB
Persistent:     yes
Autostart:      disable
Managed save:   unknown

6. Remount the cgroup filesystem:
# /etc/init.d/cgconfig start
Starting cgconfig service:                                 [  OK  ]
# /etc/init.d/libvirtd restart
Stopping libvirtd daemon:                                  [  OK  ]
Starting libvirtd daemon:                                  [  OK  ]
# lscgroup
cpuset:/
cpuset:/libvirt
cpuset:/libvirt/lxc
cpuset:/libvirt/qemu
cpu:/
cpu:/libvirt
cpu:/libvirt/lxc
cpu:/libvirt/qemu
cpuacct:/
cpuacct:/libvirt
cpuacct:/libvirt/lxc
cpuacct:/libvirt/qemu
memory:/
memory:/libvirt
memory:/libvirt/lxc
memory:/libvirt/qemu
devices:/
devices:/libvirt
devices:/libvirt/lxc
devices:/libvirt/qemu
freezer:/
freezer:/libvirt
freezer:/libvirt/lxc
freezer:/libvirt/qemu
net_cls:/
blkio:/
blkio:/libvirt
blkio:/libvirt/lxc
blkio:/libvirt/qemu

virsh # dominfo demo
Id:             17425
Name:           demo
UUID:           66c3867f-9603-daac-0d1e-48707d549823
OS Type:        exe
Persistent:     yes
Autostart:      disable
Managed save:   unknown

7. Check the guest status by virt-manager, the guest "demo" icon disappear in virt-manager and got the error log:
2011-12-23 18:13:22,776 (connection:1385): Couldn't fetch domain id '17425'
Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/connection.py", line 1383, in _update_vms
    check_new(vm, uuid)
  File "/usr/share/virt-manager/virtManager/connection.py", line 1367, in check_new
    vm = vmmDomain(self, rawvm, uuid)
  File "/usr/share/virt-manager/virtManager/domain.py", line 175, in __init__
    self._libvirt_init()
  File "/usr/share/virt-manager/virtManager/domain.py", line 216, in _libvirt_init
    self.force_update_status()
  File "/usr/share/virt-manager/virtManager/domain.py", line 1393, in force_update_status
    info = self._backend.info()
  File "/usr/lib64/python2.6/site-packages/libvirt.py", line 1407, in info
    if ret is None: raise libvirtError ('virDomainGetInfo() failed', dom=self)
libvirtError: internal error Unable to get cgroup for demo


Actual results:
As step7 results.

Expected results:
Guest name icon should not disappear in virt-manager.

Additional info:
If change the guest to under qemu, it works well.