migration: fix COLO broken caused by a previous commit

migration: fix COLO broken caused by a previous commit

This commit "migration: Create migration_is_running()" broke
COLO. Becuase there is a process broken by this commit.

colo_process_checkpoint
 ->colo_do_checkpoint_transaction
   ->migrate_set_block_enabled
     ->qmp_migrate_set_capabilities

It can be fixed by make COLO process as an exception,
Maybe we need a better way to fix it.

Cc: Juan Quintela <quintela@redhat.com>
Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>