Fix root disk icon change so that it doesn't erroneously change other icons

Summary:
I committed D16653 too soon without enough testing, because it had the effect of applying the default drive-harddisk icon to more device types. Oops. Sorry about that. :(

This patch fixes the problem, and is better tested.

Test Plan:
Other device types are not affected:

Reviewers: bruns, broulik, davidedmundson

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16678