migration: handle CANCELLING state in migration_completion()

The following sequence may cause the VM abort during migration:

1. RUN_STATE_RUNNING,MIGRATION_STATUS_ACTIVE

2. before call migration_completion(), we send migrate_cancel
   QMP command, the state machine is changed to:
     RUN_STATE_RUNNING,MIGRATION_STATUS_CANCELLING

3. call migration_completion(), and the state machine is
   switch to: RUN_STATE_RUNNING,MIGRATION_STATUS_COMPLETED

4. call migration_iteration_finish(), because the migration
   status is COMPLETED, so it will try to set the runstate
   to POSTMIGRATE, but RUNNING-->POSTMIGRATE is an invalid
   transition, so abort().

The migration_completion() should not change the migration state
to COMPLETED if it is already changed to CANCELLING.

Signed-off-by: Longpeng(Mike) <longpeng2@huawei.com>
Message-Id: <20201105091726.148-1-longpeng2@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>