KVM is a common hypervisor used by Linux servers. It offers a reasonable comparison to say EC2, without costing users thousands of dollars.
Roughly I think this is something Phabricator should support before EC2, as it provides a solid testing bed for complex Drydock interactions before we risk real user dollars.
@epriestley I have a reasonable implementation of this working on the new Drydock architecture. I want to pursue this before EC2 so we can ensure that all of the locking mechanisms are working correctly before we use the new Drydock on something that potentially costs money (and in the event that Drydock goes haywire, may cost thousands of dollars). KVM offers a safe alternative, because if Drydock goes haywire, the worst that will happen is that it won't be able to allocate new VMs on the KVM host due to out of memory errors.
There's a few minor things to sort out first (T9811 / T9812) before I submit that diff though.
Closing this off as I don't use dynamically allocated hosts any more (it's too much maintenance of custom patches).