xen: update ring.h

Update include/xen/interface/io/ring.h to its newest version.

Switch the two improper use cases of RING_HAS_UNCONSUMED_RESPONSES() to
XEN_RING_NR_UNCONSUMED_RESPONSES() in order to avoid the nasty
XEN_RING_HAS_UNCONSUMED_IS_BOOL #define.

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