Fix code after feedback, add ability to disable guest orders

Hmmm... I have check twice and my changes do not break any Backward Capability and registration process from checkout pages have same behavior: right after registration form was filled and submitted, customer will be automatically sign-in and redirected to next step page.
Also all needed events will be triggere by FosUserBundle [RegistrationController](https://github.com/FriendsOfSymfony/FOSUserBundle/blob/ef7ca325929ff102fff521653a2e3b88c7a40361/Controller/RegistrationController.php#L61)

Can you please describe you case a little bit more, because I can understand what exactly was missing.