Description of problem:
Fail to start LXC guest, the /sys/fs/cgroup directory is not exist in rhel6 kernel yet. 

Version-Release number of selected component (if applicable):
# rpm -q libvirt kernel
libvirt-0.9.13-3.el6.x86_64
kernel-2.6.32-291.el6.x86_64 

How reproducible:
100% 

Steps to Reproduce:
1. Define a LXC guest
# cat toy.xml
<domain type='lxc'>
  <name>toy</name>
  <uuid>386f5b25-43ee-9d62-4ce2-62c3809e47c1</uuid>
  <memory>500000</memory>
  <currentMemory>500000</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>destroy</on_crash>
  <devices>
    <emulator>/usr/libexec/libvirt_lxc</emulator>
    <interface type='network'>
      <source network='default'/>
    </interface>
    <console type='pty'>
      <target port='0'/>
    </console>
  </devices>
</domain>

# virsh -c lxc:/// define toy.xml
Domain toy defined from toy.xml


2. Start the LXC guest
virsh # start toy
error: Failed to start domain toy
error: internal error guest failed to start: PATH=/bin:/sbin TERM=linux container=lxc-libvirt container_uuid=d1f4798b-bebf-d93c-1d97-fe1c1cb7c780 LIBVIRT_LXC_UUID=d1f4798b-bebf-d93c-1d97-fe1c1cb7c780 LIBVIRT_LXC_NAME=toy /bin/sh
2012-07-25 07:26:47.994+0000: 1: info : libvirt version: 0.9.13, package: 3.el6 (Red Hat, Inc.<http://bugzilla.redhat.com/bugzilla>, 2012-07-20-03:24:32, x86-010.build.bos.redhat.com)
2012-07-25 07:26:47.994+0000: 1: error : lxcContainerIdentifyCGroups:1282 : Unable to read directory /sys/fs/cgroup: No such file or directory
2012-07-25 07:26:47.996+0000: 18089: info : libvirt version: 0.9.13, package: 3.el6 (Red Hat, Inc.<http://bugzilla.redhat.com/bugzilla>, 2012-07-20-03:24:32, x86-010.build.bos.redhat.com)
2012-07-25 07:26:47.996+0000: 18089: error : virCommandWait:2314 : internal error Child process (ip link set veth1 netns 18090) status unexpected: exit status 2

  
Actual results:
Fail to start with wrong cgroup dir. 

Expected results:
Start LXC domain successfully. 

Additional info:
This works on libvirt-0.9.10-21.el6.x86_64.