lxc: Inherit namespace feature

lxc: Inherit namespace feature

This patch adds feature for lxc containers to inherit namespaces.
This is very similar to what lxc-tools or docker provides.  Look
for "man lxc-start" and you will find that you can pass command
args as [ --share-[net|ipc|uts] name|pid ]. Or check out docker
networking option in which you can give --net=container:NAME_or_ID
as an option for sharing +namespace.

>From this patch you can add extra libvirt option to share
namespace in following way.

 <lxc:namespace>
   <lxc:sharenet type='netns' value='red'/>
   <lxc:shareipc type='pid' value='12345'/>
   <lxc:shareuts type='name' value='container1'/>
 </lxc:namespace>

The netns option is specific to sharenet. It can be used to
inherit from existing network namespace.

Co-authored: Daniel P. Berrange <berrange@redhat.com>