iotests: Fix use-{list,dict}-literal warnings

pylint proposes using `[]` instead of `list()` and `{}` instead of
`dict()`, because it is faster.  That seems simple enough, so heed its
advice.

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20210824153540.177128-3-hreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

tests/qemu-iotests/iotests.py

@@ -703,7 +703,7 @@ class VM(qtest.QEMUQtestMachine):
 
     def flatten_qmp_object(self, obj, output=None, basestr=''):
         if output is None:
-            output = dict()
+            output = {}
         if isinstance(obj, list):
             for i, item in enumerate(obj):
                 self.flatten_qmp_object(item, output, basestr + str(i) + '.')
@@ -716,7 +716,7 @@ class VM(qtest.QEMUQtestMachine):
 
     def qmp_to_opts(self, obj):
         obj = self.flatten_qmp_object(obj)
-        output_list = list()
+        output_list = []
         for key in obj:
             output_list += [key + '=' + obj[key]]
         return ','.join(output_list)