def user_hostname(self, vm_name=None):
        '''
        Return a string combining user and hostname, e.g. 'vagrant@127.0.0.1'.
        This string is suitable for use in an ssh commmand.  If user is None
        or empty, it will be left out of the string, e.g. 'localhost'.  If
        hostname is None, have bigger problems.

        Raises an Exception if the Vagrant box has not yet been created or
        has been destroyed.
        '''
        user = self.user(vm_name=vm_name)
        user_prefix = user + '@' if user else ''
        return user_prefix + self.hostname(vm_name=vm_name)