tools/virtio: fix build after 4.2 changes

more stubs, mostly

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
diff --git a/tools/virtio/asm/barrier.h b/tools/virtio/asm/barrier.h
index aff61e1..26b7926 100644
--- a/tools/virtio/asm/barrier.h
+++ b/tools/virtio/asm/barrier.h
@@ -3,6 +3,8 @@
 #define mb() __sync_synchronize()
 
 #define smp_mb()	mb()
+# define dma_rmb()	barrier()
+# define dma_wmb()	barrier()
 # define smp_rmb()	barrier()
 # define smp_wmb()	barrier()
 /* Weak barriers should be used. If not - it's a bug */