virtio: add ability to iterate over vqs

For cleanup it's helpful to be able to simply scan all vqs and discard
all data. Add an iterator to do that.

Cc: stable@vger.kernel.org
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/linux/virtio.h

@@ -157,6 +157,9 @@ int virtio_device_freeze(struct virtio_device *dev);
 int virtio_device_restore(struct virtio_device *dev);
 #endif
 
+#define virtio_device_for_each_vq(vdev, vq) \
+	list_for_each_entry(vq, &vdev->vqs, list)
+
 /**
  * virtio_driver - operations for a virtio I/O driver
  * @driver: underlying device driver (populate name and owner).