Retrieve a file from the device into the file-like dest_file.

def recv(self, filename, dest_file, timeout=None):
    """Retrieve a file from the device into the file-like dest_file."""
    transport = DataFilesyncTransport(self.stream)
    transport.write_data('RECV', filename, timeout)
    for data_msg in transport.read_until_done('DATA', timeout):
      dest_file.write(data_msg.data)