Initial support for Proxmox VE guest monitoring

This patch against r4386 adds preliminary support for
monitoring guests running on Proxmox VE platform. Both
OpenVZ containers and KVM virtual machines are supported.

Poller connects to Proxmox node via Proxmox's PVE2 API
and polls stats from all guests on all hosts in a cluster.
Stores RRD files under proxmox subdir for each device.

Included PVE2 API client code used by poller is taken from
https://github.com/CpuID/pve2-api-php-client.

It also adds "Proxmox VE Guests" tab on the device page
that displays CPU, Memory, Disk Usage, Disk I/O and Network
Traffic statistics for each guest.