run.py: Add --scsi option to boot from virtio-scsi

Use --scsi or -S to boot from virtio-scsi instead of virtio-blk device.

This options is default to false.

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