kvm: Publicize kvm_irqchip_release_virq

This allows to drop routes created by kvm_irqchip_add_irq/msi_route
again.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/kvm-stub.c b/kvm-stub.c
index db3a7dc..ec351d9 100644
--- a/kvm-stub.c
+++ b/kvm-stub.c
@@ -136,3 +136,7 @@
 {
     return -ENOSYS;
 }
+
+void kvm_irqchip_release_virq(KVMState *s, int virq)
+{
+}