Reduce the time of checkpoint for COLO

Reduce the time of checkpoint for COLO

we should set ram_bulk_stage to false after ram_state_init,
otherwise the bitmap will be unused in migration_bitmap_find_dirty.
all pages in ram cache will be flushed to the ram of secondary guest
for each checkpoint.

Signed-off-by: Lei Rao <[email protected]>
Signed-off-by: Derek Su <[email protected]>
Signed-off-by: Zhang Chen <[email protected]>
Reviewed-by: Li Zhijian <[email protected]>
Reviewed-by: Zhang Chen <[email protected]>
Signed-off-by: Jason Wang <[email protected]>