COLO: Introduce checkpointing protocol

COLO: Introduce checkpointing protocol

We need communications protocol of user-defined to control
the checkpointing process.

The new checkpointing request is started by Primary VM,
and the interactive process like below:

Checkpoint synchronizing points:

                   Primary               Secondary
                                            initial work
'checkpoint-ready'    <-------------------- @

'checkpoint-request'  @ -------------------->
                                            Suspend (Only in hybrid mode)
'checkpoint-reply'    <-------------------- @
                      Suspend&Save state
'vmstate-send'        @ -------------------->
                      Send state            Receive state
'vmstate-received'    <-------------------- @
                      Release packets       Load state
'vmstate-load'        <-------------------- @
                      Resume                Resume (Only in hybrid mode)

                      Start Comparing (Only in hybrid mode)
NOTE:
 1) '@' who sends the message
 2) Every sync-point is synchronized by two sides with only
    one handshake(single direction) for low-latency.
    If more strict synchronization is required, a opposite direction
    sync-point should be added.
 3) Since sync-points are single direction, the remote side may
    go forward a lot when this side just receives the sync-point.
 4) For now, we only support 'periodic' checkpoint, for which
   the Secondary VM is not running, later we will support 'hybrid' mode.

Signed-off-by: zhanghailiang <[email protected]>
Signed-off-by: Li Zhijian <[email protected]>
Signed-off-by: Gonglei <[email protected]>
Cc: Eric Blake <[email protected]>
Cc: Markus Armbruster <[email protected]>
Cc: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Reviewed-by: Amit Shah <[email protected]>
Signed-off-by: Amit Shah <[email protected]>