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>

kvm-stub.c

@@ -78,6 +78,11 @@ int kvm_has_many_ioeventfds(void)
     return 0;
 }
 
+int kvm_allows_irq0_override(void)
+{
+    return 1;
+}
+
 void kvm_setup_guest_memory(void *start, size_t size)
 {
 }