Add an explicit method for forcing emulated VGA output. Previously it
was possible only by removing `gui` feature (setting it to false had a
different effect), or enabling debug mode.
Using lack of a feature as a third state was a bad idea.
QubesOS/qubes-issues#3585
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.
