That's part of the issue, indeed. In theory, OpenStack Glance should be able to transform images from qcow2 to RAW. We tested this feature on our cluster and found it still too unreliable to enable for wider usage. It's not unlikely though that the newer version of OpenStack Glance has improved the conversion feature, thus allowing users to upload images based on QCOW2 and other formats.
Before you ask: I don't have an ETA yet for an upgrade of the cluster