avoid a bogus COMPLETED->CANCELLED transition

avoid a bogus COMPLETED->CANCELLED transition

Avoid a bogus COMPLETED->CANCELLED transition.
There is a period of time from the timing of setting COMPLETED state to that of migration thread exits, so during which it's problematic in COMPLETED->CANCELLED transition.

Signed-off-by: Zeng Junliang <zengjunliang@huawei.com>
Signed-off-by: Zhang Haoyu <haoyu.zhang@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>