Ok. I have checked my old notes.
It seems I have used a very specific procedure for the migration to Virtio.
Here is what I did.
- Use temporary devices for NIC and SCSI. I have used E1000 and Megaraid SAS, in other cases SATA disks
- install virtio drivers
- add an additional disk with virtio SCSI - this was necessary to load / install the drivers
- after that disk is recognized / installed: shutdown the OS an reconfigure to the Virtio SCSI controller and disks.
Apologies, I had it much simpler in my mind (but it is a few years back ....)