Python 3: drop usage of iter.next()

To support both Python 3 and 2, use next(iter) or iter.__next__()
Signed-off-by: NHaotong Chen <hachen@redhat.com>

qemu/tests/balloon_check.py

@@ -174,7 +174,7 @@ class BallooningTest(MemoryBaseTest):
         """
         logging.info("Wait until guest memory don't change")
         is_stable = self._mem_state()
-        utils_misc.wait_for(is_stable.next, timeout, step=10.0)
+        utils_misc.wait_for(lambda: next(is_stable), timeout, step=10.0)
 
     def get_memory_boundary(self, balloon_type=''):
         """