continue supporting scsi drives in buster

In qemu > 2.12 (e.g. buster), you can no longer use if=scsi. So any virtual guest using HDA_DRIVER=scsi will fail with:

machine type does not support if=scsi,bus=0,unit=1

The explanation for why is available and so is a helpful post with suggestions on a work-around.

The fix does not seem trivial.