LXC: enable user namespace only when user set the uidmap

LXC: enable user namespace only when user set the uidmap

User namespace will be enabled only when the idmap exist
in configuration.

If you want disable user namespace,just remove these
elements from XML.

If kernel doesn't support user namespace and idmap exist
in configuration file, libvirt lxc will start failed and
return "Kernel doesn't support user namespace" message.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>