Since QEMU 1.6 and Libvirt 1.2.3 it is possible to use auto
converge during live migrations. This option helps to live migrate
instances with memory-intensive workload by throttling down CPU
on guest machine.
Partially-implements: blueprint auto-live-migration-completion
Change-Id: I967bad47b67586ea2838e219eed1c6dce91ce153