Windows Server 2012 R2
i saw this article https://social.technet.microsoft.com/Forums/windowsserver/en-US/d145c2d4-1f40-48ec-acba-9539012d18e1/how-to-backup-a-hyperv-image?forum=winserverhyperv
but it refers to using WSB for a Windows Server 2008 R2 hyper-v installation.