drm/i915/gvt: throw error on unhandled vfio ioctls

commit 9f591ae6 upstream.

On unknown/unhandled ioctls the driver should return an error, so
userspace knows it tried to use something unsupported.

Cc: stable@vger.kernel.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -1153,7 +1153,7 @@ static long intel_vgpu_ioctl(struct mdev_device *mdev, unsigned int cmd,
 		return 0;
 	}
 
-	return 0;
+	return -ENOTTY;
 }
 
 static ssize_t