In nm_manager_get_best_device_for_connection(), not only check whether the first found
active-connection has a device. There might be multiple candidates, in which case iterate
over them and figure out which one is the most suitable.

Also, despite the found @ac has the same settings-connection, it does not
mean that the connection is available on the device. Extend the check and
only return compatible and ready devices. This can easily happen that
the settings-connection was modified in the meantime and no longer is
compatible with the device (despite currently being active on the
device, but with the previous settings).