Wouldn't hibernating the machine and then copying the hibernation data to another machine do the trick? Sure it wouldn't be as instant, but it would skip most of the boot configuration.
On virtual machines there probably isn't nearly so much real hardware to set up, and hypervisors can optimize virtual device setup. The machine image could also be provided via COW snapshot of memory. So I don't know, possibly at some corners it could be faster to do a resume type operation than a full boot. I haven't played with hypervisor guest boot at these kinds of speeds before.