NMDeviceGre and NMDeviceMacvlan didn't deal with the possibility that
the virtual device might be created before its parent's NMDevice is
created. Mostly fix this by having them put off the call to
nm_manager_get_device_by_ifindex() until someone actually requests the
device.
This is not perfect; if someone listening to notify::parent checks
right away, they may find that the parent property is still NULL, and
notify::parent will not be emitted again when it gets filled in. But
it's better than what's there now, when parent would remain NULL
forever in this case.
NMDeviceVeth did not have this problem, but it did have another
possible problem because it wasn't cleaning up its weak references
properly.
Rather than passing UDI, ifname, and driver name to the device
constructors as separate arguments, just pass the NMPlatformLink
instead and let it parse them out.
Virtual types still take UDI and ifname separately, since we create
fake NMDevices for them for autoactivating connections. That's weird
in other ways too though, so perhaps this should be revisted.
