By passing the relevant if= value to the disk we support different disk
types. The only change is that we'll translate "paravirtual" to
"virtio" to keep only one "paravirtualized" value, around ganeti. The
if= value is calculated outside the disks loop, as it's the same for all
disks (as currently ganeti doesn't support per-disk params).

Reviewed-by: iustinp