KVM: PPC: MPIC: Restrict to e500 platforms

The code as is doesn't make any sense on non-e500 platforms. Restrict it
there, so that people don't get wrong ideas on what would actually work.

This patch should get reverted as soon as it's possible to either run e500
guests on non-e500 hosts or the MPIC emulation gains support for non-e500
modes.

Signed-off-by: Alexander Graf <agraf@suse.de>