In all honesty try to avoid upgrading an existing host. Microsoft recommend a clean install and from my own experience upgrades can introduce "gremlins" that you'll spend ages trying to resolve.
Using Veeam you can migrate using this guide:http://www.veeam.com/blog/3-east-steps- ... 12-r2.html
Alternatively you can use the migration wizard in cluster manager. This doesn't require any additional storage. Assuming you have a two node cluster and have a tested backup:
1. Evacuate VM's to the other node
2. Evict the node from the cluster manager, your 2008 cluster will now be a one node cluster.
3. Rebuild the node with 2012R2, and create a new cluster (again this will be a one node cluster) If using ISCSI attach your volumes to the new node, but do not online them or anything else.
4. Run the migration wizard from 2012R2, this will populate the node with your VM's and Disks. Check that they are all there before proceeding to the next step.
5. If everything is OK, turn off your VM's and offline the disks in Cluster manager on the old cluster. Online the disks in the new cluster and start your VM's
6. Update the integration services on each VM and reboot them.
7. If everything is working as expected rebuild the other node and join to the cluster.
Before doing a migration, I'd do a quick check that your existing VM's are healthy and check for any unusual NIC configurations, such like WNLB (which uses MAC spoofing), Static MAC addresses and the like.