Description of problem:
I have a mixed cluster in RHEV 3.4: with one RHEL 7.0 host and one RHEL 6.5 host. After dealing with some VM disks, the VMs can't start on RHEL 6.5 host anymore because the disk images were upgraded to incompatible version:
2014-11-28T13:53:40.199311Z qemu-kvm: -drive <options>: 'drive-virtio-disk0' uses a qcow2 feature which is not supported by this qemu version: QCOW version 3

RHEV should instruct RHEL 7 hosts not to use such features unless some RHEL 7-only cluster level is chosen (not available yet)

Version-Release number of selected component (if applicable):
3.4.3:
RHEV-M: rhevm-backend-3.4.3-1.2.el6ev.noarch

el6 host:
vdsm-4.14.17-2.el6ev.x86_64
libvirt-0.10.2-46.el6.x86_64
qemu-kvm-rhev-0.12.1.2-2.445.el6.x86_64

el7 host:
vdsm-4.14.17-1.el7.x86_64
libvirt-1.1.1-29.el7_0.3.x86_64
qemu-kvm-rhev-1.5.3-60.el7_0.10.x86_64

How reproducible:
always

Steps to Reproduce:
1. create a new VM disk using rhel7 SPM host
2. try to start the VM on rhel6 host
3.

Actual results:
VM can't start with error above

Expected results:
VMs are backwards-compatible

Additional info: