Fix formatting in tests

Author: marmarek

qubes/tests/vm/qubesvm.py

@@ -1230,7 +1230,7 @@ def test_600_libvirt_xml_hvm_with_qemu_args(self):
         vm.netvm = None
         vm.virt_mode = "hvm"
         vm.debug = True
-        vm.features['qemu-extra-args'] = '-some-option'
+        vm.features["qemu-extra-args"] = "-some-option"
         libvirt_xml = vm.create_config_file()
         self.assertXMLEqual(
             lxml.etree.XML(libvirt_xml), lxml.etree.XML(expected)