openvmm: vmm_tests: fetch data from blobs at runtime, rather than requiring user to download them in advance

[mattkur]

One of the issues with running vmm tests is that it takes a very long time to download the images used to test. On a recent test machine over good internet, it took me 20 minutes. We have the primitives (thanks to @jstarks and @daprilik) to grab only the used sectors at runtime.

• [ ] Update our petri test infra to use SqliteDisk(BlobDisk(url)) instances, instead of FileDisk(path) instances in tests. Start with this: this gives the simple primitive that we can then evaluate / build from.
• [ ] Likely depends on a fix for #952
• [ ] Play with this some / get some feedback to see if there should be any sort of prefetch.

Reported-By: @daprilik

[mattkur]

As of 2025-09-09, tracked by Microsoft-internal Azure DevOps work item 59182520.