There were two bugs:
1. Firewall configuration wasn't copied during qvm-clone (it is in
separate file, so now it is included in vm.clone_disk_files).
2. Non-default firewall configuration wasn't stored in qubes.xml. This
means that initially DispVM got proper configuration (inherited from
calling VM), but if anything caused firewall reload (for example
starting another VM), the firewall rules was cleared to default state
(allow all).
Fixesqubesos/qubes-issues#1032