All of our CI instances (coordinators and runners) are based on Ubuntu 12.04 x64 images. You are free to install any sort of software needed to run your builds on the runners.
For example DigitalOcean's Ubuntu 12.04 x64 images work fine with Vagrant/VirtualBox, and their CPUs support VT-x. However since they do not enable VT-x in their BIOS you can only run x86 guests.
If you could let me know the output of these commands in your images before I drop $10 to try it out it would be much appreciated:
$ cat /proc/cpuinfo| grep 'vmx' # Checks for VT-x support on the CPU.
$ rdmsr 0x3a # Checks if VT-x is enabled in the BIOS.