Since qubesd properly handle chained startup of sys-net->sys-firewall
etc, we don't need a separate service to start netvm explicitly earlier.
FixesQubesOS/qubes-issues#2533
systemd-user-sessions.service is specicically for that, do not use hack
(plymouth-quit.service), which doesn't work when the service is
disabled.
FixesQubesOS/qubes-issues#1250
When user logins, login script will try to connect all guid to all the
running VMs. If VMs are still booting at this stage, will never
automatically get its guid (until user tries to start some program
there). This can for example lead to lack of nm-applet icon.
