Remove VirtualBox support in release 2.0

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I can't find a good reason to keep VirtualBox in netlab:

• I haven't seen a request to implement something for VirtualBox in years, and we only support it for a few devices.
• VirtualBox on MacOS no longer makes sense. The last x86 product was discontinued almost two years ago.
• WSL runs on Windows 10 Home edition for ~ 4 years (https://www.reddit.com/r/bashonubuntuonwindows/comments/j5c0xd/is_wsl_2_available_on_windows_10_home/)

Obviously, one could still use VirtualBox with netlab release 1.9.x (and I have no problems backporting critical fixes into a -postX release), but it would be gone from 2.0