multipass doesn't work with multipart/mixed cloud-init mime type

**Describe the bug**
I want to use multipart cloud-init configurations with multipass.

**To Reproduce**

Using [demo.txt](https://github.com/canonical/multipass/files/5716121/demo.txt):

None of the following commands seem to work
```

Start by reproducing the reported commands with demo.txt through the `multipass launch --cloud-init -` entry point. Trace how the input is parsed, then verify that multipart/mixed userdata is accepted in the supported encoding and that the launch completes without the reported YAML or scalar errors.

Written by the indexing model from the issue text.