KVM: PPC: Add Book3S compatibility code

Some code we had so far required defines and had code that was completely
Book3S_64 specific. Since we now opened book3s.c to Book3S_32 too, we need
to take care of these pieces.

So let's add some minor code where it makes sense to not go the Book3S_64
code paths and add compat defines on others.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>