1. call virtqueue_notification() only when buffer exist to avoid unnecessary
calls to virtqueue callback.
2. call virtqueue_notification() only when vq != NULL, because
rproc_virtio_notified() may be called when the virtqueues is not created:
virtqueue created in the virtio/vhost driver but the virtio devices
has been added to the remoteproc virtio devices list after the remoteproc
transport layer create the virtio devices.
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>