COLO: Remove colo_state migration struct

COLO: Remove colo_state migration struct

We need to know if migration is going into COLO state for
incoming side before start normal migration.

Instead by using the VMStateDescription to send colo_state
from source side to destination side, we use MIG_CMD_ENABLE_COLO
to indicate whether COLO is enabled or not.

Signed-off-by: zhanghailiang <[email protected]>
Signed-off-by: Zhang Chen <[email protected]>
Signed-off-by: Zhang Chen <[email protected]>
Reviewed-by: Dr. David Alan Gilbert <[email protected]>
Signed-off-by: Jason Wang <[email protected]>