[Device Notifier] Disable the possibility to unmount the root partition
Test Plan:
Reviewers: Plasma, VDG, davidedmundson, ngraham
Reviewed By: Plasma, VDG, davidedmundson, ngraham
Subscribers: abetts, ngraham, broulik, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D17402