First of all you really should be upgrading your 3.5 server so that the time to get your virtual servers back up and running is kept to a minimum. As mentioned, the virtual hardware versions are different and it will just cause you unnecessary pain getting them to work.
In terms of backups, I suggest you look at products like Veaam. It's an awesome backup tool that would have you up and running in no time at all. You can even set it to replicate a copy of the guest to the other host just in case you suffer a total loss.
I hope this helps