Thanks for trying Fusion Technology Preview 2017. We can reproduce your issue in house and have developer working to fix it.
Here is a workaround: if you have Fusion 8.x installed, you can change the hardware version of your Mac 10.13 VM to hardware version 12, then change the GOS type to macOS 10.12, after that the Mac VM can be powered on in Fusion 8.x
Thanks! I was a little skeptical that the workaround would work, but it looks like it does using the following procedure:
1. Set up a new VM, set to install from disc or image
2. Choose the High Sierra installer application.
3. Build new VM
4. Go through Apple's Setup Assistant process.
5. Encrypt the VM using System Preferences.