tools/virtio: make vringh_test use inbuf/outbuf.

As expected, the simplified accessors are faster.

for i in `seq 50`; do /usr/bin/time -f 'Wall time:%e' ./vringh_test --indirect --eventidx --parallel --fast-vringh; done 2>&1 | stats --trim-outliers:

Before:
	Using CPUS 0 and 3
	Guest: notified 0, pinged 39062-39063(39063)
	Host: notified 39062-39063(39063), pinged 0
	Wall time:1.760000-2.220000(1.789167)

After:
	Using CPUS 0 and 3
	Guest: notified 0, pinged 39037-39063(39062)
	Host: notified 39037-39063(39062), pinged 0
	Wall time:1.640000-1.810000(1.676875)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/tools/virtio/linux/virtio.h b/tools/virtio/linux/virtio.h
index 5fa612a..6df181a 100644
--- a/tools/virtio/linux/virtio.h
+++ b/tools/virtio/linux/virtio.h
@@ -63,6 +63,16 @@
 		      void *data,
 		      gfp_t gfp);
 
+int virtqueue_add_outbuf(struct virtqueue *vq,
+			 struct scatterlist sg[], unsigned int num,
+			 void *data,
+			 gfp_t gfp);
+
+int virtqueue_add_inbuf(struct virtqueue *vq,
+			struct scatterlist sg[], unsigned int num,
+			void *data,
+			gfp_t gfp);
+
 void virtqueue_kick(struct virtqueue *vq);
 
 void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);