A 9p filesystem is supported by the emulator, using a virtio transport. Using
it, files can be exchanged with the guest OS, see
[`create_file`](docs/api.md#create_filestring-file-uint8array-data-functionobject-callback)
and
[`read_file`](docs/api.md#read_filestring-file-functionobject-uint8array-callback). It can
be enabled by passing the following options to `V86Starter`:

```javascript
options.filesystem = {
    basefs: "http://localhost/9p/fs.json",
    baseurl: "http://localhost/9p/base/",
}
```

Here, `basefs` is a json file created using
[fs2json](https://github.com/copy/fs2json). The base url is the prefix of a url
from which the files are available. For instance, if the 9p filesystem has a
file `/bin/sh`, that file must be accessible from
`http://localhost/9p/base/bin/sh`. If `basefs` and `baseurl` are omitted, an
empty 9p filesystem is created.

The `mount_tag` of the 9p device is `host9p`. In order to mount it in the
guest, use:

```sh
mount -t 9p host9p /mnt/9p/
```