PPC: Add timer when running KVM

For some odd reason we sometimes hang inside KVM forever. I'd guess it's
a race condition where we actually have a level triggered interrupt, but
the infrastructure can't expose that yet, so the guest ACKs it, goes to
sleep and never gets notified that there's still an interrupt pending.

As a quick workaround, let's just wake up every 500 ms. That way we can
assure that we're always reinjecting interrupts in time.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
