Prefix disk name on size labels

Summary:
labels for CPU frequency are "CPU 1 Clock Frequency". Network speeds
have the interface name at front. Partitions didn't match, leaving the
key in the graph to be just "Available" several times if you plotted
multiple disks.

This adds a prefix so it's consistent.

BUG: 421513

Reviewers: lbeltrame, Plasma, ahiemstra

Reviewed By: ahiemstra

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29752