We went through this a while back.
Whether people like to admit it based on what is convenient or not, even HA is not covered by Windows VLK licensing. A license is assigned to one physical machine. The abillity to "fail over", or vmotion around for maintenance is provided as a feature of Software Assurance.
The loophole of course is to buy Datacenter edition for each host. Then you can pretty much do what you want.