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

nixos/lib/test-driver/test_driver/machine.py

@@ -833,6 +833,7 @@ class Machine:
             # 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)