conf: add xen type for channels
    
    So far only guestfwd and virtio were supported. Add an additional
    for Xen as libxl channels create a Xen console visible to the guest.
    
    Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
    Signed-off-by: Jim Fehlig <jfehlig@suse.com>