Revert xen/io/ring.h of "Clean up a few header guard symbols"

Revert xen/io/ring.h of "Clean up a few header guard symbols"

This reverts changes to include/hw/xen/io/ring.h from commit
37677d7db39a3c250ad661d00fb7c3b59d047b1f.

Following 37677d7db3 "Clean up a few header guard symbols", QEMU start
to fail to build:

In file included from ~/xen/tools/../tools/include/xen/io/blkif.h:31:0,
                 from ~/xen/tools/qemu-xen-dir/hw/block/xen_blkif.h:5,
                 from ~/xen/tools/qemu-xen-dir/hw/block/xen-block.c:22:
~/xen/tools/../tools/include/xen/io/ring.h:68:0: error: "__CONST_RING_SIZE" redefined [-Werror]
 #define __CONST_RING_SIZE(_s, _sz) \

In file included from ~/xen/tools/qemu-xen-dir/hw/block/xen_blkif.h:4:0,
                 from ~/xen/tools/qemu-xen-dir/hw/block/xen-block.c:22:
~/xen/tools/qemu-xen-dir/include/hw/xen/io/ring.h:66:0: note: this is the location of the previous definition
 #define __CONST_RING_SIZE(_s, _sz) \

The issue is that some public xen headers have been imported (by
f65eadb639 "xen: import ring.h from xen") but not all. With the change
in the guards symbole, the ring.h header start to be imported twice.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Message-Id: <20190621105441.3025-2-anthony.perard@citrix.com>