Route PC irqs to ISA bus instead of i8259 directly

Route PC irqs to ISA bus instead of i8259 directly

A PC has its motherboard IRQ lines connected to both the PIC and IOAPIC.
Currently, qemu routes IRQs to the PIC which then calls the IOAPIC, an
incestuous arrangement.  In order to clean this up, create a new ISA IRQ
abstraction, and have devices raise ISA IRQs (which in turn raise the i8259
IRQs as usual).

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>