Exclude all loop devices from partition and disk sensors
Since there is little reason to monitor loop devices, exclude them
altogether from the partition and disk sensors.
This fixes cases where there are a significant amount of snaps,
which are all mounted as loop devices and thus cause partition and
disk sensors to appear, which potentially causes major slowdowns.
Test Plan: Snaps are excluded from the list of sensors in KSysGuard.
Reviewers: Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: IlyaBizyaev, anthonyfieroni, jriddell, plasma-devel
Differential Revision: https://phabricator.kde.org/D29688