qubes/firewall: apply only IPv4 rules

Currently dom0 do not assign IPv6 addresses for VMs, so there is no
sense in IPv6 firewall yet.

QubesOS/qubes-issues#1815

qubes/vm/mix/net.py

@@ -269,7 +269,7 @@ def reload_firewall_for_vm(self, vm):
         # would trigger reload right away
         self.qdb.rm(base_dir)
         # write new rules
-        for key, value in vm.firewall.qdb_entries().items():
+        for key, value in vm.firewall.qdb_entries(addr_family=4).items():
             self.qdb.write(base_dir + key, value)
         # signal its done
         self.qdb.write(base_dir[:-1], '')