kvm: x86: Add user space part for in-kernel i8259

Introduce the alternative 'kvm-i8259' device model that exploits KVM
in-kernel acceleration.

The PIIX3 initialization code is furthermore extended by KVM specific
IRQ route setup. GSI injection differs in KVM mode from the user space
model. As we can dispatch ISA-range IRQs to both IOAPIC and PIC inside
the kernel, we do not need to inject them separately. This is reflected
by a KVM-specific GSI handler.

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