Remove the virtual :interface-name properties and their getters, and
use property overrides to do backward-compat handling when
serializing/deserializing.

Now when constructing an NMConnection from a hash, if the virtual
property is set and the NMSettingConnection property isn't, then the
override for NMSettingConnection:interface-name will set that property
to the value of the virtual interface-name. And when converting an
NMConnection to a hash, the overrides for the virtual properties will
return the value of NMSettingConnection:interface-name.