The previous patch didn't change the behavior when load,
it resulted in ugly code. This patch cleans it up.

With this patch, pic irq lines are manipulated when loaded.
It is expected that it won't change the behaviour because
the interrupts are level: at the moment e.g. pci devices already
reassert interrupts on load.

Test:
- rung linux as guest and use flooding ping (ping -f) to host
  in order to trigger interrupts for e1000 emulated.
- savevm/loadvm and see guest kept running after loadvm.

To be honest, I'm not sure that ping -f caused enough interrupts
because Linux e1000 driver supports NAPI.
TODO: test more OSes, stress test with save/load, live-migration

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>