Is it possible to add new users, autmatically clone a exclusivly accessible vm from a template for that user afterwords, and get a link where the user directly can log in in his exlusive machine, withouth going over the GUI?
Its possible. However you will have to write your own script/front-end to do it. Whether via API or CLI, the tools are there - bringing it all together into a workflow that does what you want will require coding by someone. Its not terribly difficult.