pci: prepare irq code for interrupt state

This rearranges code in preparation for interrupt state
implementation.
Changes:
	- split up bus walk away from interrupt handling
          into a subroutine
	- change irq_state from an array to bitmask
	- verify that irq_state values are 0 or 1 on load

There are no functional changes.

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