A "scsi-disk" is either a hard disk or a CD-ROM, depending on the
associated BlockDriverState's type hint.  Unclean; disk vs. CD belongs
to the guest part, not the host part.

Have separate qdevs "scsi-hd" and "scsi-cd" to model disk vs. CD in
the guest part.

Keep scsi-disk for backward compatibility.

Don't copy scsi-disk property removable to scsi-cd.  It's not used and
always zero(!) there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>