Disk count is limited by PCI channels in VM

[jgus]

In the current VM setup, every disk occupies an individual PCI channel (this is how virtio-blk-pci disks work) so you can maybe get about 15 disks max. That's enough for most situations, but I have deployments with 40+ disks, and I want to be able to verify correctness in VM before deploying to actual hardware.

I've already submitted a PR for this, but it hasn't gotten any attention: #1208