vm/net: fix handling netvm reset to default

If there was some netvm set, unset it first (same as with ordinary set).
Otherwise it will try to attach new netvm without detaching the old one
first.

qubes/vm/mix/net.py

@@ -317,6 +317,17 @@ class NetVMMixin(qubes.events.Emitter):
             self.untrusted_qdb.rm(mapped_ip_base + '/visible-gateway')
 
 
+    @qubes.events.handler('property-pre-del:netvm')
+    def on_property_pre_del_netvm(self, event, name, oldvalue=None):
+        ''' Sets the the NetVM to default NetVM '''
+        # pylint: disable=unused-argument
+        # we are changing to default netvm
+        newvalue = type(self).netvm.get_default(self)
+        if newvalue == oldvalue:
+            return
+        self.fire_event('property-pre-set:netvm', pre_event=True,
+            name='netvm', newvalue=newvalue, oldvalue=oldvalue)
+
     @qubes.events.handler('property-del:netvm')
     def on_property_del_netvm(self, event, name, oldvalue=None):
         ''' Sets the the NetVM to default NetVM '''