The "Mac99" type so far defines a "U2" based configuration. Unfortunately,
there have never been any U2 based PPC64 machines. That's what the U3 was
developed for.

So let's split the Mac99 machine in a PPC64 and a PPC32 machine. The PPC32
machine stays "Mac99", while the PPC64 one becomes "Mac99_U3". All peripherals
stay the same.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>