VirtualBox can't operate in VMX root mode

KVM-y and VirtualBox-y, living together in perfect harmony. Behold, a short tutorial on how to work around the VirtualBox VMX root mode error in Linux, due to loaded KVM kernel module known to cause conflict between the two hypervisors, including overview of the problem, solution via manual kernel module unloading, background information for the sudden manifestation of the issue, and more. Old problem, fresh take. Enjoy.