[Virtio-fs] [virtiofsd-rs] MR merged: Give --inode-file-handles an obligatory argument

[virtiofs-bot at sinrega.org]

What I read from issue #17 is that the first thing we must do is have `--inode-file-handles` take an obligatory (trying to avoid “mandatory�) argument and not be an alias for the new `--inode-file-handles=fallback` anymore.  We must do this before the 1.0 release.

This series does this, and also has the fix strongly suggested by Vivek, which is to return all errors from `FileHandle::to_openable()` to the guest instead of considering them a reason to fall back to an `O_PATH` FD.  I included that because this way the documentation on what `--inode-file-handles=fallback` does is final.

(Adding `--inode-file-handles=mandatory` seems trivial, but before we can do that, we need this change here anyway, so let’s start with this, and then do that.)

Tested with pjdfstest, and by reverting b9ac8c31682f42fded0a7726af5b10a170d16e88, running with `sandbox=none`, and seeing that accessing the mount in the guest fails.
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https://gitlab.com/virtio-fs/virtiofsd-rs/-/merge_requests/54