test-driver.py: defaulting keepVmState in Machine init

ecb73fd555 introduced a new keepVmState
CLI flag for test-driver.py. This CLI flags gets forwarded to the
Machine class through create_machine.

It created a regression for the boot tests where __main__ end up not
being evaluated. See
https://github.com/NixOS/nixpkgs/pull/97346#issuecomment-690951837 for
bug report.

Defaulting keepVmState to false when __main__ ends up not being
evaluated.

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

@@ -228,7 +228,7 @@ class Machine:
             return path
 
         self.state_dir = os.path.join(tmp_dir, f"vm-state-{self.name}")
-        if not args["keepVmState"]:
+        if not args.get("keepVmState", False):
             self.cleanup_statedir()
         os.makedirs(self.state_dir, mode=0o700, exist_ok=True)
         self.shared_dir = create_dir("shared-xchg")