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

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: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NAlex Williamson <alex.williamson@redhat.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>

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

@@ -1254,7 +1254,7 @@ static long intel_vgpu_ioctl(struct mdev_device *mdev, unsigned int cmd,
 
 	}
 
-	return 0;
+	return -ENOTTY;
 }
 
 static ssize_t