A "scsi-disk" device can be either a hard disk or a CD-ROM,
if there is ",media=cdrom" specified for the backend, it's
a CD-ROM, otherwise it's a hard disk.

But upstream qemu splitted "scsi-disk" into "scsi-hd" and
"scsi-cd" since commit b443ae, and ",media=cdrom" is not
required for scsi-cd anymore. "scsi-disk" is still supported
for backwards compatibility, but no doubt we should go
foward.