Reconsider virtio-fs support

**What are you trying to do?**
Currently mounting directories that get even modest activity make the entire system unusably slow.

**What's your proposed solution?**
Allow the possibility of activating `virtio-fs` for those environments that support it - like ubuntu (and mac apparently?).

**Additional context**

You [rejected this](https://github.com/canonical/multipass/issues/2735) back in 2022 and then [closed the related](https://github.com/canonical/multipass/issues/1759) ticket as "fixed"... But it's still horribly slow. Research suggests `virtio-fs` might well significantly improve the situation. My personal experience is that this is *significantly* worse than WSL2 - so if Ubuntu can't do better on Ubuntu then that would really be a great shame.

Start by reviewing the earlier issues linked in the description, especially #2735 and #1759, then investigate how Multipass currently mounts directories in the affected Ubuntu and macOS environments. Done would require a supported virtio-fs approach with its platform constraints, implementation scope, and a way to verify that directory activity no longer causes the reported slowdown.

Written by the indexing model from the issue text.