hvm: Ensure ioemu sees media-change events even when media filename

remains constant my writing a dummy blank string first.
Signed-off-by: Takanori Kasai <kasai.takanori@jp.fujitsu.com>

tools/python/xen/xend/server/blkif.py

@@ -98,6 +98,11 @@ class BlkifController(DevController):
 
         if (dev_type == 'cdrom' and new_front['device-type'] == 'cdrom' and
             dev == new_back['dev'] and mode == 'r'):
+            # dummy device
+            self.writeBackend(devid,
+                              'type', new_back['type'],
+                              'params', '')
+            # new backend-device
             self.writeBackend(devid,
                               'type', new_back['type'],
                               'params', new_back['params'])