blk: Introduce IF_TYPE_VIRTIO

This adds a new block interface type for VirtIO block devices.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>