VT-x is not enabled in the bios of our cloud images. Is there a specific need to run the build inside of a nested vm, as opposed to running directly in the runner itself? Feel free to email me jfrye [at] githost.io to discuss further.
Also, forgot to mention you can try GitHost for free. There is no upfront billing.
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.