images: add a network namespace id into images

It is possible to assign id for network namespaces and
this id will be used by the kernel in some netlink messages.
If no id is assigned when the kernel needs it, it will be
automatically assigned by the kernel.

For example, this id is reported for peer veth devices.

v2: add a comment
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>