Allow user login while some VMs are still starting. This improves UX of
the system startup, since user can start using the system earlier.
FixesQubesOS/qubes-issues#3149
1. Make sure VMs are started after dom0 actual memory usage is reported
to qmemman, otherwise dom0 will hold 4GB, even if just a little over 1GB
is needed at that time.
2. Request only vm.memory MB from qmemman, instead of vm.maxmem. While
HVM with PCI devices indeed do not support populate-on-demand, this is
already handled in libvirt XML.
The later may often cause VM startup fail on systems with 8GB of memory,
because maxmem is 4GB there and with dom0 keeping the other 4GB (see
point 1) there is not enough memory to start any sych VM.
FixesQubesOS/qubes-issues#3462
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.
