qubes/vm/net: use domain's "visible IP" for a gateway address

This is the IP known to the domain itself and downstream domains. It may
be a different one than seen be its upstream domain.

Related to QubesOS/qubes-issues#1143`

qubes/vm/mix/net.py

@@ -142,7 +142,7 @@ class NetVMMixin(qubes.events.Emitter):
     @property
     def gateway(self):
         '''Gateway for other domains that use this domain as netvm.'''
-        return self.ip if self.provides_network else None
+        return self.visible_ip if self.provides_network else None
 
     @qubes.tools.qvm_ls.column(width=15)
     @property