migration: fix counting xbzrle cache_miss_rate

Sync up xbzrle_cache_miss_prev only after migration iteration goes
forward

Signed-off-by: Xiao Guangrong <xiaoguangrong@tencent.com>
Message-Id: <20180604095520.8563-4-xiaoguangrong@tencent.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
1 file changed