[createiso] Add back running isohybrid on x86 disk images
    
    rhbz#1331317 when we refactored how we make dvds in
    df400002d8a7520609e27e8e46973342e0ab4985 we lost the ability to boot
    the dvd as a disk image.
    
    Signed-off-by: Dennis Gilmore <dennis@ausil.us>