kvm: x86: Establish IRQ0 override control

KVM is forced to disable the IRQ0 override when we run with in-kernel
irqchip but without IRQ routing support of the kernel. Set the fwcfg
value correspondingly. This aligns us with qemu-kvm.

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