kvm: x86: Prepare VCPU loop for in-kernel irqchip

Effectively no functional change yet as kvm_irqchip_in_kernel still only
returns 0, but this patch will allow qemu-kvm to adopt the VCPU loop of
upsteam KVM.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
