xen: sync include/xen/interface/io/ring.h with Xen's newest version

Sync include/xen/interface/io/ring.h with Xen's newest version in
order to get the RING_COPY_RESPONSE() and RING_RESPONSE_PROD_OVERFLOW()
macros.

Note that this will correct the wrong license info by adding the
missing original copyright notice.
Signed-off-by: Juergen Gross <jgross@suse.com>