nixos/test-driver: Crash on closed socket during connect()

If a VM crashes during `connect()`, e.g. because of `panic_on_fail`
during initrd, we would spin on the closed socket forever. We should
rasie an exception instead.

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

@@ -819,9 +819,8 @@ class Machine:
 
             while True:
                 chunk = self.shell.recv(1024)
-                # No need to print empty strings, it means we are waiting.
                 if len(chunk) == 0:
-                    continue
+                    raise RuntimeError("Shell disconnected")
                 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: