kvm: Fix build for non-CAP_IRQ_ROUTING targets

kvm: Fix build for non-CAP_IRQ_ROUTING targets

A type definition and a KVMState field initialization escaped the
required wrapping with KVM_CAP_IRQ_ROUTING. Also, we need to provide a
dummy kvm_irqchip_release_virq as virtio-pci references (but does not
use) it.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Ben Collins <bcollins@ubuntu.com>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>