nixos/test-driver: log what to do if backdoor service doesn't come oneline

            # TODO: do we want to bail after a set number of attempts?

            while not shell_ready(timeout_secs=30):

                self.log("Guest root shell did not produce any data yet...")

                self.log("  To debug, enter the VM and run 'systemctl status backdoor.service'.")

            while True:

                chunk = self.shell.recv(1024)