Via QMP, migrate -b (and -i) is available in our RHEL-6.x packages, but this option is not supported and was introduced by accident. The proper way to migrate a VM without shared storage is using drive_mirror and NBD, as supported in the -rhev flavor of QEMU.
virsh migrate --live --copy-storage-{all,inc} currently calls migrate -b in RHEL6.x and apparently it automatically calls block_stream if it's available (for example, in RHEV). This BZ will be cloned against libvirt to make sure these options are properly handled after the removal of migrate -b.
Please note, that blindly removing blk and inc arguments of migrate QMP command is not going to work well with libvirt. Libvirt always sets these arguments (to either false or true). If you really have to remove this functionality, reporting a suitable error when any of those arguments are true would be the best solution from libvirt's point of view.