dissect-image: move comment to right place

dissect-image: move comment to right place

The image name is extracted from the image path originally passed in,
i.e. not the contents of the image. And the image UUID is directly
retrieved from the partition table, hence also not from the contents.
Let's hence move the comment to separate out the stuff extract from the
file systems (and thus only available when mounting/with privs/with
block devices) from the data available without any of that.