piix_pci: optimize set irq path

optimize irq routing in piix_pic.c which has been a TODO.
So far piix3 tracks each pirq level and checks whether a given pic pins is
asserted by seeing if each pirq is mapped into the pic pin.
This is independent on irq routing, but data path is on slow path.

Given that irq routing is rarely changed and asserting pic pins is on
data path, the path that asserts pic pins should be optimized and
chainging irq routing should be on slow path.
The new behavior with this patch series is to use bitmap which is addressed
by pirq and pic pins with a given irq routing.
When pirq is asserted, the bitmap is set and see if the pic pins is
asserted by checking the bitmaps.
When irq routing is changed, rebuild the bitmap and re-assert pic pins.

test:
- create VM with 4 e1000 nics in different pci slots
  (i.e. fn=0 for each e1000)
  Thus those e1000's INTA are connected to each PIRQ[A-D].
- run linux as guest and saw each devices triggers interrupt
  by seeing /proc/interrupts. And then confirmed that each PIRQ[A-D]
  surely asserted interrupts.
  Because irq 10 and 11 are shared by 4 e1000's, it only one NIC is activated
  with ifconfig ethN up/down when counting interrupts.

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