wait_for_login() related update

Redhat ID: 1080834



Hey Feng,
Since the latest comment was add in July, I don't know the status of this PR. If you still want to merge it into master, I'd suggest you thinking about this part of code, the propose of this lines is to provide a stable session even though the network is unstable, but the login would still fail if the restart_guest_network function failed, though it's rare, it  would happen when meeting nic driver problem and will cause non-network-related case failed.