Add isa-fdc (disables default_floppy).
Add ide-drive (disables default_cdrom).

Also walk the -global QemuOpts, so we'll catch
-global isa-fdc.drive{A,B}=<name> too.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>