test: dm: virtio: Test notify before del_vqs

The virtqueue is passed to virtio_notify() so move the virtqueue
deletion to the end of the test when it's no longer needed. This wasn't
causing any problems because the sandbox virtio transport driver doesn't
do anything for notifications, but it could cause problems if things
change and it was a bad example.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>