KVM: PIC: call ack notifiers for irqs that are dropped form irr

After commit 242ec97c PIT interrupts are no longer delivered after
PIC reset. It happens because PIT injects interrupt only if previous one
was acked, but since on PIC reset it is dropped from irr it will never
be delivered and hence acknowledged. Fix that by calling ack notifier on
PIC reset.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>