KVM: PPC: Make Performance Counters work

When we get a performance counter interrupt we need to route it on to the
Linux handler after we got out of the guest context. We also need to tell
our handling code that this particular interrupt doesn't need treatment.

So let's add those two bits in, making perf work while having a KVM guest
running.

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