KVM: Add method to check for backwards-compatible API extensions

Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index 5d24203..39cf8fd 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -2416,6 +2416,12 @@
 		r = 0;
 		break;
 	}
+	case KVM_CHECK_EXTENSION:
+		/*
+		 * No extensions defined at present.
+		 */
+		r = 0;
+		break;
 	default:
 		;
 	}