machine-image: rework error handling

machine-image: rework error handling

Let's rework error handling a bit in image_find() and friends: when we
can't find an image, return -ENOENT rather than 0. That's better as
before we violated the usual rule in our codebase that return parameters
are initialized when the return value is >= 0 and otherwise not touched.

This also makes enumeration and validation a bit more strict: we'll only
accept ".raw" as suffix for regular files, and filter out this suffix
handling on directories/subvolumes, where it makes no sense.