xen: Fix ring.h header

The xen_[rw]?mb() macros defined in ring.h can't be used and the fact
that there are gated behind __XEN_INTERFACE_VERSION__ means that it
needs to be defined somewhere. QEMU doesn't implement interfaces with
the Xen hypervisor so defining __XEN_INTERFACE_VERSION__ is pointless.
This leads to:
    include/hw/xen/io/ring.h:47:5: error: "__XEN_INTERFACE_VERSION__"
        is not defined, evaluates to 0 [-Werror=undef]

Cleanup ring.h. The xen_*mb() macros are already defined in xenctrl.h
which is included in xen_common.h.

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190704153605.4140-1-anthony.perard@citrix.com>
[aperard: Adding the comment proposed upstream]
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>