virtio-scsi: Share the same vblk name with virtio-blk

Now, virtio-scsi disks use the same name as virtio-blk, e.g., vblk0,
vlbk1. If both scsi and blk are added to guest, they share the global disk
index number.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
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>
run.py: Use newer -device and -drive syntax for virtio-blk device

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