migration : enable auto-converge capability v2

This reduce guest cpu speed if dirtied bytes is 50% more than the approx.amount of bytes that just got transferred since the last time we were in this routine.

qemu commit :
http://git.qemu.org/?p=qemu.git;a=commit;h=bde1e2ec2176c363c1783bf8887b6b1beb08dfee

tested with "stress -m 2 -c 2" under debian

without autoconvergence : downtime 12s - duration 12min
with autoconvergence : downtime 2s - duration 4min

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>