virsh: Introduce virshDomainInterfaceCompleter

virsh: Introduce virshDomainInterfaceCompleter

For given domain fetch list of defined interfaces. This can be
used for commands like domif-getlink and others. If available,
the interface name is returned (e.g. "vnet0", usually available
only for running domains), if not the MAC address is returned.
Moreover, the detach-interface command requires only MAC address
and therefore we have new flag that forces the completer to
return just the MAC address.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>