A. Licensing can be a pain point with VDI, and some people struggle to understand what's required. The process has gotten simpler recently, though. At minimum, you need Software Assurance (SA) on your client licenses, or to purchase Virtual Enterprise Centralized Desktop (VECD) if you don't have SA. In addition, if you're using Microsoft components for VDI, you also need Remote Desktop Services (RDS) CALs for each user or device. Even if you use the Citrix solution and use Hyper-V (and any RDS services, such as the Virtualization Host component or RemoteFX, when it ships) you still need RDS CALs.
The simplest solution is to buy the VDI licenses that Microsoft now offers. They include all the key components you need, such as System Center Virtual Machine Manager, System Center Configuration Manager, System Center Operations Manager, the RDS CAL, and the Microsoft Desktop Optimization Pack. All you need in addition are the client licenses/SA/VECD to cover the client OS. A premium version of the VDI suite exists that includes a full RDS CAL, which can be used to connect to a RDS Session Hos,t enabling presentation virtualization. The standard version of the VDI suite includes a RDS CAL to be used for VDI only.
Microsoft offers a page with more information on licensing options.