acceptance/tests/vnc.py: use explicit syntax for enabling passwords

This matches the command line on 82a17d1d, where the "on" or "off"
should be explicitly given.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210924185506.2542588-9-crosa@redhat.com>

tests/acceptance/vnc.py

@@ -45,7 +45,7 @@ def test_change_password_requires_a_password(self):
                          'Could not set password')
 
     def test_change_password(self):
-        self.vm.add_args('-nodefaults', '-S', '-vnc', ':0,password')
+        self.vm.add_args('-nodefaults', '-S', '-vnc', ':0,password=on')
         self.vm.launch()
         self.assertTrue(self.vm.qmp('query-vnc')['return']['enabled'])
         set_password_response = self.vm.qmp('change-vnc-password',