virtio-blk: Use WITH_LOCK in virtio_blk::make_virtio_request()

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
virtio-blk: Fix minor coding style issues

Fix few minor but annoying coding style issues in drivers/virtio-blk.cc.

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