rpm: fix misleading systemd warnings during upgrade

systemctl preset output lengthy warning when trying to operate on
non-existing unit. This preset action is meant to disable unit, so it's
even better it doesn't exists.

rpm_spec/core-vm.spec

@@ -600,9 +600,13 @@ else
     done
     /bin/systemctl --no-reload preset qubes-update-check.timer
     # Upgrade path - now qubes-iptables is used instead
+    if [ -f /lib/systemd/system/iptables.service ]; then
         /bin/systemctl --no-reload preset iptables.service
+    fi
+    if [ -f /lib/systemd/system/ip6tables.service ]; then
         /bin/systemctl --no-reload preset ip6tables.service
     fi
+fi
 
 # Set default "runlevel"
 rm -f /etc/systemd/system/default.target