It's a way to go when you have few guests all supporting UPS tools.Send the UPS shutdown signals to the guests first, then the host.
Anyway, it's major bug and it should be solved. I have tested PVE 4.0 month ago and after reporting this bug I installed ESXi instead of PVE. For containers I still can use CentOS/OpenVZ or FreeBSD/jails inside VM on top of ESXi.
Now I can see that this bug was reported earlier for PVE 4.0 B2 - http://forum.proxmox.com/threads/23...uests-don-t-shutdown-cleanly-on-host-shutdown