i8259: Factor out base class for KVM reuse

i8259: Factor out base class for KVM reuse

Analogously to the APIC, we will reuse some parts of the user space
i8259 model for KVM. The base class provides a common device state, the
vmstate, the property list, a reset core and some shared init bits.

This also introduces a common helper to instantiate a single i8259 chip
from the cascade-creating i8259_init function.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>