Simply navigate to that link, scroll to the bottom, find where it says 'installable binary package?' and follow that link. MAKE SURE TO INSTALL THIS IN YOUR HOST MACHINE, NOT INSIDE VBOX. The second solution is to simply rescale the host resolution to a lower one, so VirtualBox can function. You should notice immediate improvement in usability. A quick and easy workaround is to plug your computer into a secondary non-HiDPI monitor and move the virtual machine guest window to that screen.
This results in very laggy window movement, typing, and overall updating of the screen to any changes.
