Windows guests taking forever to shut down... just sitting at "Shutting down.."

I have what I'd imagine is a pretty solid RHEV setup:

RHEV-M is running on an HA virtual machine under a standard RHEL host with qemu/KVM.
I have three RHEV-H boxes, each with either 64 or 96 GB of ram and 2 x 6 core Westmere procs (x5650)
Two HP Lefthand replicated Sans @ 8TB
Entire storage network is separate from VM traffic, etc, and is contained to one GB switch.

Everything seems peachy... I am able to build and access VMs, configure virtual networking, etc. I have two domain controllers in this environment that work fine with the one domain controller that is running on VMWare.

The issue I'm coming across (and have from basically the beginning) is that Windows VM's seem to hang at the shutdown sequence. I have one that has been sitting at "Shutting down" for over an hour. The little blue circle that usually rotates is not rotating. The RHEV-M console shows that this VM is now using 0% mem and 2% CPU. As far as I can see, it is doing nothing.

What could be happening that is causing this? "quick reboots" end up taking hours, unless I force the machine down, which I can't imagine is a good thing!