prep: qdev'ify Raven host bridge (SysBus)

prep: qdev'ify Raven host bridge (SysBus)

Drop pci_prep_init() in favor of extended device state. Inspired by
patches from Hervé and Alex.

Assign the 4 IRQs from the board after device instantiation. This moves
the knowledge out of prep_pci and allows for future machines with
different IRQ wiring (IBM 40P). Suggested by Alex.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Reviewed-by: Alexander Graf <agraf@suse.de>
Cc: Hervé Poussineau <hpoussin@reactos.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>