KVM: PPC: booke: rework rescheduling checks

Instead of checking whether we should reschedule only when we exited
due to an interrupt, let's always check before entering the guest back
again. This gets the target more in line with the other archs.

Also while at it, generalize the whole thing so that eventually we could
have a single kvmppc_prepare_to_enter function for all ppc targets that
does signal and reschedule checking for us.

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