propolis factory: might want to use pci-nvme instead of pci-virtio-block?

I noticed the root disk for Propolis workers ends up being pci-virtio-block which definitely has not had attention in line with the last few months of nvme in Propolis. I don't know if it's so much different than nvme that it would matter much for CI jobs, but I don't know it's not important.

(I basically just see this as an excuse to stand up the bmat bits locally again and try out a real job with pci-nvme to see the difference. but if anyone else wants to try.. !)

a quick check on my workstation suggests that i can get upwards of 9k writes/sec (4k) with pci-nvme whereas pci-virtio-block hit a ceiling at around 7k writes/sec. throughput is frighteningly poor imo in both cases, but I can say that 'cause that's my problem :)

Start in factory/propolis/src/vm.rs at the root-disk configuration around lines 369-374, then restore the bmat setup locally and run a real job using pci-nvme. Compare it with the current pci-virtio-block behavior and document whether the change is suitable for Propolis workers; done means the disk choice and observed CI impact are clear.