pci_bus_irqs() coupled together the assignment of pci_set_irq_fn and
pci_map_irq_fn to a PCI bus. This coupling gets in the way when the
pci_map_irq_fn is board-specific while the pci_set_irq_fn is device-
specific.

For example, both of QEMU's PIIX south bridge models have different
pci_map_irq_fn implementations which are board-specific rather than
device-specific. These implementations should therefore reside in board
code. The pci_set_irq_fn's, however, should stay in the device models
because they access memory internal to the model.

Factoring out pci_bus_map_irqs() from pci_bus_irqs() allows the
assignments to be decoupled, resolving the problem described above.

Note also how pci_vpb_realize() which gets touched in this commit
assigns different pci_map_irq_fn's depending on the board.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230109172347.1830-5-shentey@gmail.com>
[PMD: Factor out in vfu_object_set_bus_irq()]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>