1) My virtual machine loses activation after upgrading to Windows 10 BootCamp based virtual machine from previous OS, however Windows is activated under native BootCamp.
2) Can't validate: error 0xc004e016 appears when activating upgraded Windows 10 BootCamp based virtual machine.
Hardware is slightly different between native Mac hardware and virtual machine hardware.
1) Go to https://www.microsoft.com
2) Log in under your Microsoft account that was used to confirm Windows 10 upgrade.
3) Locate your license key on your account.
4) Reset the activation on your computer.
5) Use Windows 10 license key to activate your Windows 10 BootCamp virtual machine.
At the end of this process both native BootCamp and the virtual machine should be activated.
Note: in case of any issues with license key reset, contact Microsoft support for assistance.