Yes you are right, but GCE does not offer the flexibility that allow user to reboot themselves in their own convenient time.

Note: I am not saying the 'live migration' in GCE does not work, I am just saying I've more confident (well...peace of mind, IMO) to shutdown the my own database manually (which is automated and tested), ensure all data are flushed to disk, clients are disconnected gracefully and slave has promoted to master.. and things like that, rather than some black magic like 'live migration'

Of course, I am not saying 'live migration' is wrong, but it is just my preference.