iotests/245: Fix type for iothread property

iothread is a string property, so None (= JSON null) is not a valid
value for it. Pass the empty string instead to get the default iothread.

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Tested-by: Peter Krempa <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

Author: kevmw

tests/qemu-iotests/245

@@ -1189,10 +1189,10 @@ class TestBlockdevReopen(iotests.QMPTestCase):
         self.run_test_iothreads('iothread0', 'iothread0')
 
     def test_iothreads_switch_backing(self):
-        self.run_test_iothreads('iothread0', None)
+        self.run_test_iothreads('iothread0', '')
 
     def test_iothreads_switch_overlay(self):
-        self.run_test_iothreads(None, 'iothread0')
+        self.run_test_iothreads('', 'iothread0')
 
 if __name__ == '__main__':
     iotests.activate_logging()