As I said in https://github.com/autotest/virt-test/pull/1379#discussion_r9107473, this method should belong to VirshSession in virsh.py rather than ShellSession. 

But I am wondering if we really need this method for it?? This method is not used so widely that we should add a exit_session in VirshSession class.

* If we add this method into VirshSession for closing the session, we have already close() here to close session.
* If we add this method into VirshSession for execute 'exit' and 'quit' in a virsh session, we should call command("exit") or command('quit'). Or we can add a module function in virsh.py named exit() and quit() to call command() same with the other virsh command, such as virsh.start().