KVM: lapic: do not set KVM_REQ_EVENT unnecessarily on PPR update

On PPR update, we set KVM_REQ_EVENT unconditionally anytime PPR is lowered.
But we can take into account IRR here already.

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>