KVM: x86: Hide KVM_IRQCHIP_KERNEL behind CONFIG_KVM_IOAPIC=y

Enumerate KVM_IRQCHIP_KERNEL if and only if support for an in-kernel I/O
APIC is enabled, as all usage is likewise guarded by CONFIG_KVM_IOAPIC=y.

Link: https://patch.msgid.link/20251206004311.479939-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/kvm_host.h

@@ -1222,7 +1222,9 @@ struct kvm_xen {
 
 enum kvm_irqchip_mode {
 	KVM_IRQCHIP_NONE,
+#ifdef CONFIG_KVM_IOAPIC
 	KVM_IRQCHIP_KERNEL,       /* created with KVM_CREATE_IRQCHIP */
+#endif
 	KVM_IRQCHIP_SPLIT,        /* created with KVM_CAP_SPLIT_IRQCHIP */
 };