libnl: Prepare for nested messages (veth creation)

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
net: Implement veth net model

It's perfrctly possible to create veth with peer end right in
the target pid namespace. One only need to properly prepare the
netlink message.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
test: Test for veth net model

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>