hw/isa/piix4: Use qdev_get_gpio_in_named() to get ISA IRQ

Since commit 078778c5 ("piix4: Add an i8259 Interrupt Controller")
the TYPE_PIIX4_PCI_DEVICE exposes the ISA input IRQs as "isa" alias.

Use this alias to get IRQ for the power management PCI function.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210324182902.692419-1-f4bug@amsat.org>