vm/systemd: fix ProxyVM related services deps (#578)

vm-systemd/qubes-firewall.service

@@ -1,6 +1,7 @@
 [Unit]
 Description=Qubes firewall updater
 ConditionPathExists=/var/run/qubes-service/qubes-firewall
+After=qubes-network.service
 
 [Service]
 ExecStart=/usr/sbin/qubes_firewall

vm-systemd/qubes-network.service

@@ -3,6 +3,7 @@ Names=qubes_firewall.service
 Description=Qubes network forwarding setup
 ConditionPathExists=/var/run/qubes-service/qubes-network
 Before=network.target
+After=iptables.service
 
 [Service]
 Type=oneshot