Constantify function parameters

Summary: Even though constantifying these parameters won't change anything(since they are being passed by value), it is done to assure they conform to rest of the coding style

Reviewers: stikonas, cjlcarvalho, KDE Partition Manager

Reviewed By: cjlcarvalho

Subscribers: KDE Partition Manager

Tags: KDE Partition Manager

Differential Revision: https://phabricator.kde.org/D20106