Refactoring to support SMS API

We are developing a vmware extension on top of pyvmomi, and we need support for the Storage Monitoring Service API (See #35).
We have already added support for that API on our wrapper for pyvmomi (you can see our repository here: https://github.com/infinidat/infi.pyvmomi_wrapper).
The following changes are needed for the support of the extra API.