Cause: RHEL 6.3 picked up an early version of block copy, prior to upstream acceptance of the feature. In the time since then, several improvements were made to the upstream feature.
Consequence: Older libvirt was unable to fully manage the block copy features present in this release of qemu.
Fix: The block copy code was finally approved upstream in both qemu and libvirt, and then libvirt was improved to work around the differences in the early implementation so that it can now work with all of RHEL 6.3, RHEL 6.4, and upstream qemu implementations.
Result: Block copy as a means of backing up the disk images of a running domain is more powerful, and has been tested to have no regressions over the support available in RHEL 6.3.