With 1 CPU and 128 MB of RAM. Sure, it's a small system, but it's enough to be useful for some purposes -- more useful than medical discoveries in mice, at least.
"Boot" isn't even a sensible concept in a VM, IMHO, you could just toss an image into RAM. There's no hardware to initialize, the caches are probably already warm, etc.
You do need to shake hands with the virtio devices; that's a bit easier than most hardware, but it's not trivial.
The "take a snapshot after you finish booting and resume that" approach can work (Lambda does it) but it only works if you have the same type and number of CPUs, the same amount of RAM, the same filesystem on disk, and even the same MAC addresses on your network interfaces. So it's not like FreeBSD can ship useful "pre-booted" images.