On devices that have no real permanent hardware address (as returned
by ethtool), we take the current MAC address of the device.

Currently, NM is a bit flaky about whether to accept such fake permanent
addresses for settings like keyfile.unmanaged-devices or the per-
connection property ethernet.mac-address. Probably, we should allow
using fake addresses there in general.

However, that leads to problems because NetworkManager itself changes
the current MAC address of such devices. For example when
configuing

  keyfile.unmanaged-device=22:33:44:55:66:77

and later activating a connection with

  ethernet.cloned-mac-address=22:33:44:55:66:77

we have a strange situation after restart and the device becomes
unmanaged.

We are going to avoid that, by remembering the fake permanent address
in the device state file.

This only matters:

  - for devices that don't have a real permanent address (veth)

  - if the user or NetworkManager itself changed the MAC address
    of the device

  - after a restart of NetworkManager, without reboot. A reboot
    clears the device state for /var/run/NetworkManager.