device: add veth support

device: add veth support

The veth config code mostly handles the primary interface of a veth pair,
the secondary interface is not explicitly referenced and will be found as
an unrelated interface after the pair has been created.

This doesn't only allow us to keep the veth code simple (and similar to
existing device handlers), but will also avoid complicating handling
unnecessarily in case the secondary interface is moved into another network
namespace.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>