xen_piix3_set_irq() isn't PIIX specific: PIIX is a single PCI device
while xen_piix3_set_irq() maps multiple PCI devices to their respective
IRQs, which is board-specific. Rename xen_piix3_set_irq() to communicate
this.

Also rename XEN_PIIX_NUM_PIRQS to XEN_IOAPIC_NUM_PIRQS since the Xen's
IOAPIC rather than PIIX has this many interrupt routes.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Tested-by: Chuck Zmudzinski <brchuckz@aol.com>
Message-Id: <20230312120221.99183-2-shentey@gmail.com>
Message-Id: <20230403074124.3925-2-shentey@gmail.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>