So that Bluetooth can use them. They used to be NMDevice subclasses, but
we need them to be generic objects that both bluetooth and the normal
modem stack can use. All because GObject can't do multiple inheritance,
but that would probably be even messier.
So now that we have generic modem objects, we can create the actual
NMDevice subclasses that will wrap them for non-BT modems, and then
also have NMDeviceBt wrap them too for DUN.
The only thing that doesn't work yet is the system-settings service's
"auto eth" connections for ethernet devices that don't have an existing
connection. Might also have issues with unmanaged devices that can't
provide a MAC address until they are brought up, but we'll see.