KVM: Future-proof the exit information union ABI

Note that as the size of struct kvm_run is not part of the ABI, we can add
things at the end.
Signed-off-by: Avi Kivity <avi@qumranet.com>

include/linux/kvm.h

@@ -111,6 +111,8 @@ struct kvm_run {
 			__u32 longmode;
 			__u32 pad;
 		} hypercall;
+		/* Fix the size of the union. */
+		char padding[256];
 	};
 };