Comment 73 for bug 787091

@Hendrik,

I suspect you left libvirt running while unmount the cgroup filesystems. Libvirt prepares and caches info about the mounted cgrousp when it starts up, and cannot handle changes.

Please try instead:

stop libvirt-bin
umount -a -t cgroup
start libvirt-bin

But actually, before you do that, coudl you tell me how the cgroups were mounted? Do you have the cgroup-bin or cgroup-lite package installed, or is it manually mounted with fstab? ('dpkg -l | grep cgroup' and 'cgroup cgroup /etc/fstab' should help)