get_devices
Description:
Gets all the known network devices.
 Use nm_device_get_type or the NM_IS_DEVICE_XXXX functions to determine what kind of device member of the returned array
      is, and then you may use device-specific methods such as 
        get_hw_address.
Parameters:
| this | a Client | 
Returns:
| a GenericArray containing all the Devices. The returned array is owned by the Client object and should not be modified. |