Don't fail the whole process when "just" appmenus import fails.
But if data import fails, remove the VM
Also update for vm.run_service_for_stdio raising CalledProcessError.
Enable 'qrexec' VM feature to wait for qrexec initialization - it is
required to call qubes.PostInstall service. If VM start fails, assume
there is no qrexec and drop that feature.
