The ISA bus IRQ range is 0..15. What isa_irq_handler and IsaIrqState are
actually dealing with are the Global System Interrupts. Refactor the
code to clarify this.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>