xen/xenbus: remove unused xenbus_map_ring()

xenbus_map_ring() is used nowhere in the tree, remove it.
xenbus_unmap_ring() is used only locally, so make it static and move it
up.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>