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>