virtio: Add virtio::probe() helper

Add a new virtio::probe() helper function to simplify virtio driver
probing.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
virtio-net: Use virtio::probe() helper

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
virtio-blk: Use virtio::probe() helper

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>