[libvirt] Make systemd work with LXC user namespaces

These two patches fix the issue that control groups are unusable if user namespaces are enabled. We have to chown() the control group to the correct user. As the container mounts the control group and only the controller is allowed to chown() the mount point we need a new barrier to synchronize them after the container has setup the control groups. Thanks, //richard [PATCH 1/2] lxc: Add another barrier [PATCH 2/2] lxc: Add virCgroupSetOwner()