Q: Is it true that SP1 must be installed for Windows Server 2008 R2 to be supported as aR2 Hyper-V guest?
A: Yes. Windows Server 2008 R2 without SP1 is no longer a supported operating system and wasn't supported when Windows Server 2012 R2 was released. Therefore, Windows Server 2008 without SP1 isn't supported as a Windows Server 2012 R2 Hyper-V guest. At the time of writing, Windows Server 2008 R2 SP1 is a supported operating system.
For more information, check out the following Microsoft resources: