Add Windows Update Manager Support for pre-XS 7.0 VMs for XenTools Updates

The ability to use the Windows Update Manager is restricted only to new VMs created/provisioned under XS 7.0. Please add the feature to be able to do the same with older VMs from earlier XS versions or VMs imported via XenConvert or other versions desired to be imported from non-XS hypervisors so that they can be maintained the same way. Handling those separately is very inconvenient and inconsistent, especially if VMs are shuffled around between XenServer hosts and pools a lot. There are also still occasionally glitches with old XenTools installs, so this newer management mechanism is much preferred.