That way of snapshotting that you used is done by:
virsh --connect $LIBVIRT_URI snapshot-create-as $DOMAIN --name $SNAP_ID
This does work on qcow2 images, but in the case of Ceph, images are in raw format. See also this topic:
If you look under the tab “storage” there is also a snapshot function, that should work properly with Ceph & raw images.
EDIT: added screenshot for clarification - see the small camera icon
