Each network namespaces has a list of ID-s for other namespaces,
so if we request infomation about a veth device, we get an id
for a namespace of a peer device.

These ID-s can be set by users or by kernel when they are required.
CRIU has to restore these ID-s for network namespaces. We have to
remember that one netns can have different id-s in different network
namespaces.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>