A: Failover Clustering hasn't allowed a rolling upgrade with a mix of OSs in a single cluster for several versions now. This is due to radical differences and improvements with each new version of Failover Clustering.
When you add to that the major changes in Hyper-V between Windows Server 2008 R2 and, it becomes clear that a mix of server versions in a single cluster wouldn't be plausible. What will be provided is a migration wizard to help make the migration of the turned-off virtual machines (VMs) from the Windows Server 2008 R2 cluster to the Windows Server 2012 cluster very fast and simple. However, because the VMs need to be turned off during the migration, there is a small amount of downtime that you will need to plan for in the migration plan.