schema: Allow lxc:namepsace children to appear individually

schema: Allow lxc:namepsace children to appear individually

Since its introduction in v1.2.19-rc1~8 our schema mandates that
LXC domain namespace child elements appear either all three at
once or not at all:

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

This is not mandated by our parser though. Neither by code that
later uses it (virLXCProcessSetupNamespaces()). Relax the schema.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>