Nova doesn't _have_ to copy image from Ceph, but does indeed do so because the following patch didn't make the feature freeze cut-off date for Havana:
https://review.openstack.org/#/c/46879/

A WIP branch with this and other related fixes can be found here:
https://github.com/angdraug/nova/compare/openstack:stable/havana...angdraug:havana-rbd-live-migration