Exclude all loop devices from partition and disk sensors

Summary:
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

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29688