KVM: PPC: Booke: only prepare to enter when we enter

So far, we've always called prepare_to_enter even when all we did was return
to the host. This patch changes that semantic to only call prepare_to_enter
when we actually want to get back into the guest.

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