If there is no 'VirtualBox.xml-prev', or if its also a 0-bytes file, then dont worry, its still fixable.
If you haven't one, just install a fresh system to mount your system on it. Delete the 'VirtualBox.xml' file (the 0-bytes one) and rename the 'VirtualBox.xml-prev' to 'VirtualBox.xml'. The solution was so simple: mount the system partition on a working system - if you have one.