[Proxmox Extension] GetHostStats returns null

### The required feature described as a wish

Just something I noticed when setting it up; proxmox doesn't seem to support returning GetHostStats information (including CPU and Memory total/used) at the moment.

Start at the Proxmox extension entry point for GetHostStats and determine whether Proxmox exposes CPU and memory total/used values. Review the existing CloudStack host-stat handling and define completion as returning those values instead of null, if the upstream capability exists; otherwise document the limitation.

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