COLO: Optimize memory back-up process

COLO: Optimize memory back-up process

This patch will reduce the downtime of VM for the initial process,
Previously, we copied all these memory in preparing stage of COLO
while we need to stop VM, which is a time-consuming process.
Here we optimize it by a trick, back-up every page while in migration
process while COLO is enabled, though it affects the speed of the
migration, but it obviously reduce the downtime of back-up all SVM'S
memory in COLO preparing stage.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Message-Id: <20200224065414.36524-5-zhang.zhanghailiang@huawei.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  minor typo fixes