[StorageAccess] Fix storageAccessFromPath returning unmounted filesystems

Appending a separator makes the empty path returned for e.g. an unmounted
filesystem indistinguishable from the filesystem root `/`.

Instead of adding more checks and fixups simplify the match to only
suceed if the given path matches the mountpoint exactly, or the trailing,
unmatched part of the path is a new path component.

Replace QDir::separator with '/', as Qt normalizes file paths to use
'/' as separators, see QFileInfo::canonicalFilePath

Make sure this case is also covered by tests.