qubes-anaconda-addon: initialize default-fw-netvm

It should be set to default netvm for proxy VMs: sys-net.

Reported by @adrelanos
Fixes QubesOS/qubes-issues#3190

qubes-anaconda-addon/org_qubes_os_initial_setup/gui/spokes/qubes_os.py

@@ -551,6 +551,7 @@ class QubesOsSpoke(FirstbootOnlySpokeMixIn, NormalSpoke):
 
         self.run_command(['/usr/bin/qvm-prefs', 'sys-firewall', 'netvm', 'sys-net'])
         self.run_command(['/usr/bin/qubes-prefs', 'default-netvm', default_netvm])
+        self.run_command(['/usr/bin/qubes-prefs', 'default-fw-netvm', 'sys-net'])
         self.run_command(['/usr/bin/qubes-prefs', 'updatevm', updatevm])
         self.run_command(['/usr/bin/qubes-prefs', 'clockvm', 'sys-net'])
         self.run_command(['/usr/bin/qvm-start', default_netvm])