virtio: explicit enable_cb/disable_cb rather than callback return.

It seems that virtio_net wants to disable callbacks (interrupts) before
calling netif_rx_schedule(), so we can't use the return value to do so.

Rename "restart" to "cb_enable" and introduce "cb_disable" hook: callback
now returns void, rather than a boolean.

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

include/linux/virtio_config.h

@@ -61,7 +61,7 @@ struct virtio_config_ops
 	void (*set_status)(struct virtio_device *vdev, u8 status);
 	struct virtqueue *(*find_vq)(struct virtio_device *vdev,
 				     unsigned index,
-				     bool (*callback)(struct virtqueue *));
+				     void (*callback)(struct virtqueue *));
 	void (*del_vq)(struct virtqueue *vq);
 };