Add support for selecting agent based on the hypervisor tag

[andreeaflorescu]

Right now we only support selecting a buildkite agent to run the job based on the os (either windows or linux) and the platform tag (x86_64.metal or arm.metal). We need to add support for another agent tag as well so that the mshv CI can use the autogeneration tool to generate the tests.

Here is an example of the tags: https://github.com/rust-vmm/mshv/blob/1a9ca01801e78f8ddc2630660cbab3bc3300e39e/.buildkite/pipeline.yml#L43

The code that needs to be updated to support this is: https://github.com/rust-vmm/rust-vmm-ci/blob/main/.buildkite/autogenerate_pipeline.py#L211. We need to do a get for hypevisor, and set it in the agents tags. The code should be similar to the way we set the platform.

[andreeaflorescu]

Fixed by #111