live migration supports xbzrle compress

By using XBZRLE (Xor Binary Zero Run-Length-Encoding) we can reduce VM downtime and total live-migration time of VMs running memory write intensive workloads typical of large enterprise applications such as SAP ERP Systems, and generally speaking for any application with a sparse memory update pattern.

On the sender side XBZRLE is used as a compact delta encoding of page updates, retrieving the old page content from an LRU cache (default size of 64 MB). The receiving side uses the existing page content and XBZRLE to decode the new page content.

This is the xbzrle introduce.
https://github.com/qemu/qemu/blob/master/docs/xbzrle.txt

also we can see there is a great improve xbzrle cache hit by ChenLiang <email address hidden>
http://git.greensocs.com/fkonrad/mttcg/commit/27af7d6ea5015e5ef1f7985eab94a8a218267a2b

This feature is ready for nova to support in live migration.