* Allow mounting a single directory without a name

To utilize `VZSingleDirectoryShare` which seems more stable than `VZMultipleDirectoryShare`.

We've been having reports from users that mounted directories occasionally return "no such file" errors when building large projects. I took a stab at reproducing the issue by running https://github.com/devMEremenko/XcodeBenchmark in a mounted directory:

```bash
tart run --dir=workdir-test:~/workspace-temp/XcodeBenchmark ventura-xcode
```

And I was able to reproduce the "no such file" error on the first try! After looking into the issue I decided to try `VZSingleDirectoryShare` as this PR changes and to my pleasant surprise it all worked like a charm the next run. So it seems there is a bug in `VZMultipleDirectoryShare` integration with virtiofs. Since in most cases users only mount a single directory it makes sense to allow doing it wihtout providing a `name`.

So now it will be possible to run the following command:

```bash
tart run --dir=~/workspace-temp/XcodeBenchmark ventura-xcode
```

Which will make `~/workspace-temp/XcodeBenchmark` available under `/Volumes/My Shared Files/` without any intermediate directories.

* Reformat

* Updated description