virtio: console: Disable callbacks for virtqueues at start of S4 freeze

To ensure we don't receive any more interrupts from the host after we
enter the freeze function, disable all vq interrupts.

There wasn't any problem seen due to this in tests, but applying this
patch makes the freeze case more robust.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -1854,10 +1854,18 @@ static int virtcons_freeze(struct virtio_device *vdev)
 
 	vdev->config->reset(vdev);
 
+	virtqueue_disable_cb(portdev->c_ivq);
 	cancel_work_sync(&portdev->control_work);
+	/*
+	 * Once more: if control_work_handler() was running, it would
+	 * enable the cb as the last step.
+	 */
+	virtqueue_disable_cb(portdev->c_ivq);
 	remove_controlq_data(portdev);
 
 	list_for_each_entry(port, &portdev->ports, list) {
+		virtqueue_disable_cb(port->in_vq);
+		virtqueue_disable_cb(port->out_vq);
 		/*
 		 * We'll ask the host later if the new invocation has
 		 * the port opened or closed.