hw/isa/piix4: Replace some magic IRQ constants

hw/isa/piix4: Replace some magic IRQ constants

This is a follow-up on patch "malta: Move PCI interrupt handling from
gt64xxx_pci to piix4". gt64xxx_pci used magic constants, and probably
didn't want to use piix4-specific constants. Now that the interrupt
handing resides in piix4, its constants can be used.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20220217101924.15347-7-shentey@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>