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>