iotests: add VM.add_object()

The VM.add_object() method can be used to add IOThreads or memory
backend objects.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20171207201320.19284-5-stefanha@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/iotests.py

@@ -197,6 +197,11 @@ class VM(qtest.QEMUQtestMachine):
                                  socket_scm_helper=socket_scm_helper)
         self._num_drives = 0
 
+    def add_object(self, opts):
+        self._args.append('-object')
+        self._args.append(opts)
+        return self
+
     def add_device(self, opts):
         self._args.append('-device')
         self._args.append(opts)