How do I completely uninstall Virtualbox from macOS?
The correct method to uninstall Virtualbox is to use the official uninstall script that is included on the installation image.  If you don't have the installation image anymore, you can re-download it. Since the program installs many helper scripts outside the /Applications folder, you need to clean those up with the included uninstall tool and not just trash the main app. 

This information is available directly from the project page as well:
https://www.virtualbox.org/manual/ch02.html#install-mac-uninstall