COLO: Don't process failover request while loading VM's state

We should not do failover work while the main thread is loading
VM's state. Otherwise the consistent of VM's memory and
device state will be broken.

We will restart the loading process after jump over the stage,
The new failover status 'RELAUNCH' will help to record if we
need to restart the process.

Cc: Eric Blake <eblake@redhat.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <1484657864-21708-4-git-send-email-zhang.zhanghailiang@huawei.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
   Added a missing '(Since 2.9)'