Add new module - OVFUtils

The bulk of this code is taken verbatim from RHEVMHelper.

It provides objects used to manipulate various bits of OVF/OVA
components in a reusable manner.
Refactor RHEVMHelper to use OVFUtils

All of the details surrounding OVF structure/generation/etc are now
available in OVFUtils, so consume that.
Add Vsphere support to OVFUtils
Add OVA plugin

This plugin takes an existing vsphere or rhevm target image, and
generates a new target image which is an OVA file valid for the
respective target.
Move OVFUtils.py from core imgfac to ovfcommon pseudo-plugin
Merge pull request #303 from jeckersb/ova_final

OVA generation support