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>

include/hw/xen/interface/io/ring.h

@@ -33,6 +33,13 @@
  * - standard integers types (uint8_t, uint16_t, etc)
  * They are provided by stdint.h of the standard headers.
  *
+ * Before using the different macros, you need to provide the following
+ * macros:
+ * - xen_mb()  a memory barrier
+ * - xen_rmb() a read memory barrier
+ * - xen_wmb() a write memory barrier
+ * Example of those can be found in xenctrl.h.
+ *
  * In addition, if you intend to use the FLEX macros, you also need to
  * provide the following, before invoking the FLEX macros:
  * - size_t
@@ -42,12 +49,6 @@
  * and grant_table.h from the Xen public headers.
  */
 
-#if __XEN_INTERFACE_VERSION__ < 0x00030208
-#define xen_mb()  mb()
-#define xen_rmb() rmb()
-#define xen_wmb() wmb()
-#endif
-
 typedef unsigned int RING_IDX;
 
 /* Round a 32-bit unsigned constant down to the nearest power of two. */