block migration of qcow2 VMs copies all empty space

I'm running openstack 2012.1 'icehouse' which, ultimately, calls down into qemu-system-x86 2.0.0+dfsg-2ubuntu1.10.

I primed the process by copying all necessary base images onto the destination host.  Nonetheless, post-migration instances are much larger than the original image; the copy duplicated all the empty space that ought to have remained copy-on-write.

It looks like there was some attempt to address this issue in the past (e.g. https://bugzilla.redhat.com/show_bug.cgi?id=1058173 ) but perhaps I'm conflating two different issues.