Add an integration test or a simple example for running FreeBSD guests

[mtjhrc]

We currently support running FreeBSD guests (as used by https://github.com/nohajc/anylinuxfs) but don't have a smoke test under the tests directory or a documented example to boot FreeBSD.

This would be quite helpful in improving support.

@nohajc

[nohajc]

You can assign this to me.

[nohajc]

I would actually like to get #480 merged too after I resolve the merge conflicts.

[nohajc]

Ok, my plan is to start with Linux/FreeBSD x86_64 and wait for the aarch64 tier 2 support to land in Rust so that nightly is not needed for compiling the guest agent.

I never actually tested on x86_64, so this will be interesting. :)

EDIT: Just remembered we actually need the PVH boot protocol: #284. That PR was never merged though.

In general, it is easier to boot FreeBSD VM on aarch64. You can use the generic kernel configuration and boot directly.

For x86_64, there's only the ELF kernel available with PVH and we might also need special configuration like the one for Firecracker unless EFI is used.

[mtjhrc]

FYI keep in mind the GitHub hosted macOS CI runner does not support running VMs, but we also have a aarch64 Linux runner that does support VMs. Even though the CI doesn't support the integration tests I do regularly run the suite locally on macOS when working on features and reviewing PRs.

[nohajc]

Yeah, I'm aware. No virtualized macOS on Apple Silicon supports nested virtualization yet. I suppose one could connect a self-hosted macOS runner to GitHub actions but Linux aarch64 should be fine.

Anyway, I already have the tests implemented and running locally on my Mac. Testing boot and virtio-net with gvproxy. I'll open a PR after I check it works on Linux too.

It should all work after #636 is merged (with the Makefile fix).

[nohajc]

I'm also trying to get this merged to gvproxy: containers/gvisor-tap-vsock#646
I assumed I would be able to use it for tests on both macOS and Linux but it turns out the unixgram support is only compiled for Darwin. When I build it with my patch, all the tests pass.

UPDATE: already merged, will be included in the next release

[nohajc]

PR: #646