vm/qubesvm: convert firewall_conf into dumb, read-only property

Don't allow anything else than firewall.xml.

qubes/tests/vm/qubesvm.py

@@ -340,20 +340,6 @@ class TC_90_QubesVM(QubesVMTestsMixin,qubes.tests.QubesTestCase):
         vm = self.get_vm()
         self._test_generic_bool_property(vm, 'include_in_backups', True)
 
-    def test_240_firewall_conf(self):
-        vm = self.get_vm()
-        self.assertPropertyDefaultValue(vm, 'firewall_conf', 'firewall.xml')
-        self.assertPropertyValue(vm, 'firewall_conf', 'other.xml',
-            'other.xml', 'other.xml')
-        del vm.firewall_conf
-        self.assertPropertyDefaultValue(vm, 'firewall_conf',
-            'firewall.xml')
-
-    @unittest.expectedFailure
-    def test_241_firewall_conf_invalid(self):
-        vm = self.get_vm()
-        self.assertPropertyInvalidValue(vm, 'firewall_conf', None)
-
     @qubes.tests.skipUnlessDom0
     def test_250_kernel(self):
         kernels = os.listdir(os.path.join(

qubes/vm/mix/net.py

@@ -91,8 +91,10 @@ class NetVMMixin(qubes.events.Emitter):
         doc='''If this domain can act as network provider (formerly known as
             NetVM or ProxyVM)''')
 
-    firewall_conf = qubes.property('firewall_conf', type=str,
-        default='firewall.xml')
+
+    @property
+    def firewall_conf(self):
+        return 'firewall.xml'
 
     #
     # used in networked appvms or proxyvms (netvm is not None)