KB3035583, which is the one that gives you the icon on the taskbar for reserving the upgrade. If you have that, but it's not showing on the taskbar, try the Susan Bradley routine you'll find here: http://answers.microsoft.com/en-us/windows/forum/windows_10-win_upgrade/i-want-to-reserve-my-free-copy-of-windows-10-but-i/848b5cce-958b-49ae-a132-a999a883265b. One of those tricks should get it to behave.
If you get the icon, and reserve a copy, it will be interesting to see if Microsoft will let you download and install it in a virtual environment. My guess is, you still may have some trouble at that point, but I'm not sure.