Currently, ethernet-based VLANs can specify the hardware address of
the parent device (and, in theory, the cloned hardware address and MTU
of the VLAN device) by using an NMSettingWired in addition to the
NMSettingVlan.

The theory was that non-ethernet-based VLANs, when we eventually
supported them, would likewise use the setting type corresponding to
their parent device. However, this turns out to be both complicated
(the settings plugins and connection editor would have a
hard-to-impossible time figuring out which setting type to use in some
cases) and incorrect (for most L2 settings [eg, BSSID, bond mode,
etc], the VLAN can't have its own values separate from the parent
device).

What we should have done was just have :mac-address,
:cloned-mac-address, and :mtu properties on NMSettingVlan. However, at
this point, for backward-compatibility, we will just stick with using
a combination of NMSettingVlan and NMSettingWired, but we will use
NMSettingWired regardless of the underlying hardware type.