I wanted to share the following customer Q&A in case you run into this yourself.
Q: I own Windows Server 2008 R2 Datacenter Edition for each physical processor in my Hyper-V host, which provides unlimited guest virtual machine (VM) rights. What license key do I use to activate the individual guest VM if we aren’t using a KMS server?
A: Go to the Microsoft Volume License Service Center and locate the Product Keys section. Find your specific enrollment (there may be more than one) and look in the Product column for a product titled “Windows Srv 2008R2 Data Ctr/Itanium - MAK C”. The key should be listed to the right of the product title. Use this key to activate Windows Server 2008 R2 Web, Standard, and Enterprise Edition guest VMs via MAK.
Note: The above scenario applies to Microsoft Volume License customers. OEM and retail scenarios may differ – so please consult your Microsoft rep or reseller for details. As with all things related to licensing and product use rights – please refer to the Microsoft Product Use Rights (PUR) document for more information.