OpenBSD: disk-based DataSource detection is unreliable

# Bug report

in ds-identify, OpenBSD disk detection doesn't exist, which means it uses the Linux paths, which don't work.
in the Python code, the disk-detection looks rather questionable:

https://github.com/canonical/cloud-init/blob/4f60ff099eb0a6f57595706f4d34cf713d3c02b1/cloudinit/util.py#L1444-L1458

none of the `criteria` are actually considered.
We only return any found `cd` drives, whether those have anything mounted or not.
But a source could just as well be mounted as disk.

## Steps to reproduce the problem

- mount a vfat formatted drive with user-data
- set `datasource_list` to NoCloud or ConfigDrive, depending on your format
- `cloud-init clean -slr`
- does it work?

## Environment details
- Cloud-init version: main
- Operating System Distribution: OpenBSD
- Cloud provider, platform or installer type: source