KVM: PIC: Only commit irq routing when necessary

The current logic updates KVM's view of our interrupt map every time we
change it. While this is nice and bullet proof, it slows things down
badly for me. QEMU spends about 3 seconds on every start telling KVM what
news it has on its routing maps.

Instead, let's just synchronize the whole irq routing map as a whole when
we're done constructing it. For things that change during runtime, we can
still update the routing table on demand.

Signed-off-by: Alexander Graf <agraf@suse.de>