The new object type represents tunnels over IPv4 and IPv6.
We have a single setting type (NMSettingIPTunnel) for tunnels and it
can't be shared among different device factories. So we define also a
single device type for all tunnels.
This new object will also represent GRE tunnels, which before were
instantiated as NMDeviceGre and had a ".Device.Gre" D-Bus
interface. This commit introduces a change in behavior.