images: regfile -- Save file @mode into image

We will need it for NFS. While we're saving it
into image, the test for mode change must be
done in account with NFS. So put fixme there.

CC: Pavel Emelyanov <xemul@virtuozzo.com>
CC: Stanislav Kinsburskiy <skinsbursky@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>