Is it possible to use a "Westmere" Mac Pro to run virual machines with FreeNAS?
In response to your question:


  Has anyone been able to get FreeNAS 11 running VMs or any other VMs to
  run on a Mac Pro with this CPU configuration (I'm not sure if being
  able to run Parallels is any indication of the ability to run VMs on
  FreeNAS).


I don't know why anyone would want to deploy a NAS solution for the purpose of creating a host to run VM but the ability of a machine to be able to host VMs is essentially dependent on two factors:


CPU capabilities
OS Support



CPU Support

As for the Westmere processors - they are fully capable of supporting Virtualization technology.1  From their webpage:

Advanced Technologies

  Intel® Turbo Boost Technology                                  1.0
  Intel® Hyper-Threading Technology                              Yes
  Intel® Virtualization Technology (VT-x)                        Yes
  Intel® Virtualization Technology for Directed I/O (VT-d)       Yes
  Intel® VT-x with Extended Page Tables (EPT)                    Yes




O/S Support

FreeNAS is based on the FreeBSD operating system which uses the BSD-licensed bhyve Hypervisor.2  So, because it's supported in FreeBSD, FreeNAS which is based on FreeBSD supports virtualization.

TL;DR

Yes, FreeNAS 11 on your Westmere processors supports virtualization though I don't think you will find very many folks who have deployed FreeNAS for the purpose of virtualization as it primarily a storage centric product.  

If your main goal here is to built a virtualization platform, find a OS and Hypervisor that fits your needs.  If you just want a good (free) OS that supports Virtualization go for FreeBSD.



1 Intel® Xeon® Processor E5620

2 FreeBSD as a Host with bhyve