virtio: sandbox: Bind RNG rather than block device

The virtio-rng driver is extremely simple, making it suitable for
testing more of the virtio uclass logic. Have the sandbox driver bind
the virtio-rng driver rather than the virtio-blk driver so it can be
used in tests.

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