Currently there's no workaround, we have to wait until this bug receives the attention from developers and hopefully the next VritualBox build will resolve this issue.
When you see that your VM has frozen on reboot, simply press `Host_key + R` or via the menu -> Machine -> Reset (it's safe).
I've added an attachment which serves as a manifestation
Keywords: VirtualBox, Windows 10 preview, hang, freeze, installation.