qvm-start-gui lifecycle should be bound to X server lifecycle. It should
be restarted when user logoff and login again, at least to start
gui-daemons again.
Do that by opening a connection to X server and reacting to breaking
that socket.
FixesQubesOS/qubes-issues#3147
Very few calls at client side really needs VM class name. So, even in
non-blind mode use just QubesVM class, to avoid strange cases depending
on blind mode being enabled or not. Then, have VM class name in 'klass'
property. If known at object creation time, cache it, otherwise query
qubesd at first access.
Don't start GUI daemon for given VM twice when qvm-start-gui was started
during VM startup (while waiting for qrexec startup). This is especially
common while running tests.
Report failed qubes.SetMonitorLayout as warning (instead of unhandled
exception).
Clear VM cache on qubesd reconnect.
Fix logging.
