hw/pci: removed irq field from PCIDevice

Instead of exposing the the irq field,
pci wrappers to qemu_set_irq or qemu_irq_*
can be used.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
