Version 0.12.2 of qemu-kvm was released earlier this week
. This version continues to be derived from the upstream QEMU code-base with various KVM enhancements. For the most part it's various fixes making up the qemu-kvm 0.12.2 release, but there is a new feature and that is block migration.
The block migration support in qemu-kvm 0.12.2 makes it possible to do block migration of a virtual disk during the live migration process. Using this feature during a live QEMU migration will copy the entire virtual disk image asynchronously and then go back and copy the dirty blocks that were modified since the initial copy. This feature supports both live migration with complete storage copy and live migration with incremental storage copy.
Details concerning the block migration feature for qemu-kvm 0.12.2 can be found at Linux-KVM.com
along with the migration instructions.