migration: final handshake for the resume

migration: final handshake for the resume

Finish the last step to do the final handshake for the recovery.

First source sends one MIG_CMD_RESUME to dst, telling that source is
ready to resume.

Then, dest replies with MIG_RP_MSG_RESUME_ACK to source, telling that
dest is ready to resume (after switch to postcopy-active state).

When source received the RESUME_ACK, it switches its state to
postcopy-active, and finally the recovery is completed.

Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Peter Xu <[email protected]>
Message-Id: <20180502104740[email protected]>
Signed-off-by: Juan Quintela <[email protected]>