KVM: PPC: Pass init/destroy vm and prepare/commit memory region ops down

This arranges for the top-level arch/powerpc/kvm/powerpc.c file to
pass down some of the calls it gets to the lower-level subarchitecture
specific code.  The lower-level implementations (in booke.c and book3s.c)
are no-ops.  The coming book3s_hv.c will need this.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>