virtio: Add transport feature handling stub for virtio_ring.

To prepare for virtio_ring transport feature bits, hook in a call in
all the users to manipulate them.  This currently just clears all the
bits, since it doesn't understand any features.

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

drivers/virtio/virtio_pci.c

@@ -99,6 +99,9 @@ static void vp_finalize_features(struct virtio_device *vdev)
 {
 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
 
+	/* Give virtio_ring a chance to accept features. */
+	vring_transport_features(vdev);
+
 	/* We only support 32 feature bits. */
 	BUILD_BUG_ON(ARRAY_SIZE(vdev->features) != 1);
 	iowrite32(vdev->features[0], vp_dev->ioaddr+VIRTIO_PCI_GUEST_FEATURES);