Thanks for your comments.

Here vm.wait_for_login will not need long time.  Most time it is one time login. just like vm.login().

The main reason Use vm.wait_for_login() to create new session is to make sure the session is always active.

We repeat reboot for many times.  So this case need long time.  If we only create session at beginning. Then we have to check its available before using it.  

So I create new session when using it. It help make sure guest always alive and session is always active. Or we can get clear error message.