[Virtio-fs] [virtiofsd-rs] MR merged: Implement xattr name mapping

[virtiofs-bot at sinrega.org]

Closes #3 

These patches are an implementation of the [xattr name mapping specification](https://gitlab.com/qemu-project/qemu/-/blob/master/docs/tools/virtiofsd.rst) which is held in the QEMU tree.

I also performed a fair bit of manual testing with various rules in addition to the automated unit tests which exercise map parsing and rule executions in isolation so I could see how it all worked end-to-end. To tell you the truth, I did not have any experience with extended attributes or using xattr mappings with virtio-fs prior to starting this work, so any additional test cases or reviews for correctness are sincerely appreciated.
---
https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/9