pci.c: restore bus-level pci interrupt state via pci_set_irq

pci.c: restore bus-level pci interrupt state via pci_set_irq

This change abolishes pcibus_save.  Instead we use an invariant - that
device interrupts are supposed to be reflected in the bus interrupt
state - to restore the bus interrupt state.

This makes the code smaller and removes one way in which a savefile
could be corrupted (eg, if it had been generated by a buggy emulator).

It also means that systems which do some of their own PCI bus
emulation and thus reflect PCI bus state elsewhere (eg CPU acclerators
such as Xen) get notified of the PCI bus interrupt level, via
pci_set_irq's call to bus->set_irq.