hw/pxb: add map_irq func

The bios does not index the pxb slot number when
it computes the IRQ because it resides on bus 0
and not on the current bus.
However Qemu routes the irq through bus 0 and adds
the pxb slot to the IRQ computation of the PXB device.

Synchronize between bios and Qemu by canceling
pxb's effect.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>