get_identifier
Description:
Gets the identifier of the given kind for this.
The only identifier currently available is g_drive_identifier_kind_unix_device.
Parameters:
| this | 
           a Drive  | 
      
| kind | 
           the kind of identifier to return  | 
      
Returns:
| 
           a newly allocated string containing the requested identifier, or null if the Drive doesn't have this kind of identifier.  |