Merge pull request #242315 from K900/fix-uefi-backdoor

            while not shell_ready(timeout_secs=30):

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

            self.log(self.shell.recv(1024).decode())

            while True:

                chunk = self.shell.recv(1024)

                self.log(f"Guest shell says: {chunk!r}")

                # NOTE: for this to work, nothing must be printed after this line!

                if b"Spawning backdoor root shell..." in chunk:

                    break

            toc = time.time()

            self.log("connected to guest root shell")