You might also want to consider Oracle Sun servers. Oracle is making a strategic move into storage for surveillance. I would expect to see a focus on more support for VMS integration to DB/OS management tools from them.
If you are going to run a virtualized machine environment you definitely want to do it using a bare metal hypervisor like Xen, VMWare Sphere or ESX, or KVM. Using a hosted hypervisor is going to be less efficient, less secure, and lower performance. I like VirtualBox, but it has that OS host which always poses a security risk.
We have run Milestone, Exacq, our MVE system, and a few others on Xen under windows 7 pro 64 guest OS with good results, both on a local server and in a remote hosted server. These were simple servers that were just writing H.264 encoded streams right out to disk and play back from disk. So a server for archive is easy to run on almost any VM environment.
If you want to serve video out to clients, then that is a different thing.
If you are going to transcode for the purpose of serving up rate adapted/device adapted playback, ala DASH or something like the HauteSpot MVE system, then you will want to use something like the NVIDIA VGX or Intel Quick Sync drivers for your host system. NVidia has H.264 encoding in hardware on all of their CUDA GPUs and Intel has Quick Sync in all Ivy Bridge processors. You will need your VM hypervisor and guest drivers to support the GPU too. Citrix Xen and VMWare both support NVIDIA. I don't know of any VM that supports Intel Quick Sync yet. Support means letting virtual guests os have access to the GPU for acelleration. So today if there were a VMS that was optimized to use the NVIDIA GPU under virtualization, then everything would be awesome. Adobe has already built in this to their software. Right now I don't know of any VMS that has drivers to support this. Network Optix supports the Intel Quick Sync hardware acelleration, but not under virtualization.
With NVIDIA VGX acelleration you should be able to transcode up to 8 30fps H.264 1080p (2MP) streams. But I don't know of any VMS client viewing architecture built around this.
Most of the development of GPUs in virtualization has been for graphic workstations not for streaming/transcoding. If you want to serve to many client systems (like in a Security Operations Center) folks like Citrix have built optimized VM Clients that have graphics support. They use XWindows or RDP like remote graphics calls to display their guest os on a remote client. Another applications of virtualization would be for remote desktops like this. It does not take care of transcoding or rate adaptation but it would allow you to have many desktops sharing one server. Of course then you have a single point of failure. But the trade off is lower cost workstations and network security.
It would be interesting to know if anyone is working on optimizing their VMS to use GPU acelleration like the NVIDIA VGX in transcoding for streaming out to remote clients. If anyone knows more about off-loading encoding to the GPU in virtualized environments I would love to hear about it.