migration/migration.c: Add missed default error handler for migration state

In the migration_completion() no other status is expected, for
example MIGRATION_STATUS_CANCELLING, MIGRATION_STATUS_CANCELLED, etc.

Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/migration.c

@@ -3205,7 +3205,7 @@ static void migration_completion(MigrationState *s)
         qemu_mutex_unlock_iothread();
 
         trace_migration_completion_postcopy_end_after_complete();
-    } else if (s->state == MIGRATION_STATUS_CANCELLING) {
+    } else {
         goto fail;
     }