Werk #11786: proxmox_ve_disk_usage: use same metrics and summary as df checks

proxmox_ve_disk_usage summary and metrics have been very sparse, not including FS size or percentage of usage. This changes makes the check output look like other df based checks including "fs_used", "fs_size" and "fs_used_percent" metrics, as well as a summary in form of "50.00% used (1.10 TB of 2.20 TB)".

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