test: dm: virtio: Test virtio device driver probing

Once the virtio-rng driver has been bound, probe it to trigger the pre
and post child probe hooks of the virtio uclass driver. Check the status
of the virtio device to confirm it reached the expected state.

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