Fix bug in create_if_needed

Closes #12.

The logic is as follows:

* If the FS is "none", there is NOTHING to format. Period.
* If the FS is something else, we format it IFF it the block device doesn't have it yet.

As discussed with @stdweird on Friday, the new feature to enforce the filesystem (e.g, to (optionally) re-format an ext4 device into XFS if the profile states so) is a different improvement, that will be welcome when it gets tests.