virtio: support unlocked queue kick

Based on patch by Christoph for virtio_blk speedup:

	Split virtqueue_kick to be able to do the actual notification
	outside the lock protecting the virtqueue.  This patch was
	originally done by Stefan Hajnoczi, but I can't find the
	original one anymore and had to recreated it from memory.
	Pointers to the original or corrections for the commit message
	are welcome.

Stefan's patch was here:

	https://github.com/stefanha/linux/commit/a6d06644e3a58e57a774e77d7dc34c4a5a2e7496
	http://www.spinics.net/lists/linux-virtualization/msg14616.html

Third time's the charm!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/virtio.h

@@ -34,6 +34,10 @@ int virtqueue_add_buf(struct virtqueue *vq,
 
 void virtqueue_kick(struct virtqueue *vq);
 
+bool virtqueue_kick_prepare(struct virtqueue *vq);
+
+void virtqueue_notify(struct virtqueue *vq);
+
 void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
 
 void virtqueue_disable_cb(struct virtqueue *vq);