You've repeatedly claimed it, but the logic doesn't hold. KVM is a dependency for Firecracker, just like brotli is a dependency for Node. What matters isn't "who's according-to-Hoyle responsible", what matters is "are you exposed to it".

Not for nothing, but it would have been possible for Firecracker to disable the partial checksum feature this vulnerability exploits. If you wanted to push on the argument a bit, you could say that Firecracker ran virtio-net in an aggressive and possibly unsafe configuration. I wouldn't agree, but it's a colorable argument.