tools/virtio: stub out strong barriers

The tool should never use them, abort if it does.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/virtio/linux/virtio.h b/tools/virtio/linux/virtio.h
index b4fbc91..7579f19 100644
--- a/tools/virtio/linux/virtio.h
+++ b/tools/virtio/linux/virtio.h
@@ -181,6 +181,9 @@
 #define smp_mb()	mb()
 # define smp_rmb()	barrier()
 # define smp_wmb()	barrier()
+/* Weak barriers should be used. If not - it's a bug */
+# define rmb()	abort()
+# define wmb()	abort()
 #else
 #error Please fill in barrier macros
 #endif