Fix "VirtualBox can't operate in VMX root mode" [Linux]

Yesterday, after trying out TestDrive in Ubuntu, which installs QEMU, I got this error when trying to open a virtual machine in VirtualBox:

VirtualBox can't operate in VMX root mode

Luckily, there is an easy fix for Linux users. All you have to do is run the following command with root privileges (the command has also been added to the Ubuntu TestDrive post, of course):

modprobe -r kvm_intel

For Ubuntu, just use "sudo" in front of the command. Unfortunately I don't know a fix for Windows users other than just using a single virtualization application (so no more than one!).

Hopefully this will help some people. Using it, you can run both Ubuntu TestDrive (which uses QEMU) and VirtualBox without any issues.