A "ide-drive" 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.

Upstream qemu splitted "ide-drive" into "ide-hd" and "ide-cd"
since commit 1f56e32, and ",media=cdrom" is not required for
ide-cd anymore. "ide-drive" is still supported for backwards
compatibility, but no doubt we should go foward.