xen: import new ring macros in ring.h

Sync the ring.h file with upstream Xen, to introduce the new ring macros.
They will be used by the Xen transport for 9pfs.

CC: konrad.wilk@oracle.com
CC: boris.ostrovsky@oracle.com
CC: jgross@suse.com
CC: groug@kaod.org

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Acked-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>