netapp plugin: Added support for additional statistics.

These commits add support for the following statistics:

* SnapVault® lag and transfer stats
* Quota (qtree based disk / file usage) stats
* more detailled SIS stats (dedup / compress saved)

Also, support for VFilers has been added. Any metric may now also be queried in the context of a VFiler unit. This is useful, for example, for SnapVault stats since the host system does not provide the information of SnapVaults targetting a VFiler.