Readme.md and virtio-net

[Rucadi]

Hello,

Just recently I discovered this amazing library, so I was investigating PR and commits and so on, and found out that virtio-net was introduced past month with: https://github.com/containers/libkrun/pull/142

And the networking part of the README.md is not updated, or at least is not clear enough to me if TSI is still being used or if virtio-net is available, as it specifically says that it's not.

Since I'm not so familiar yet, I'll not create a PR with a corrected README, but just wanted you to notice :)

Thanks for this amazing piece of software!