As I am doing some tests with qemu, I realized that the way it does 'migrate -i tcp:DEST:444' is not the same as 'libvirt migrate --copy-storage-inc'. Basically qemu uses the same stream as RAM migration and libvirt takes advantage of NBD transfer.
With virsh migrate-setspeed I observed that one can only control the transfer throughput of RAM, but not disk synchronization. At least this is what I can see in bmon when doing a migration with incremental copy.
The question is: Am I missing something or it is not implemented?
Thank you guys!