KVM: PPC: Fix error in BAT assignment

BATs didn't work. Well, they did, but only up to BAT3. As soon as we
came to BAT4 the offset calculation was screwed up and we ended up
overwriting BAT0-3.

Fortunately, Linux hasn't been using BAT4+. It's still a good
idea to write correct code though.

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