In Qubes the disposable netVM is internet connected. For this reason,
on Qubes we chose create our own disposable VM (dz-dvm). However, in
reality this could still be bypassed since dz-dvm had the default
disposable dispvm.
By setting the default_dispvm to '' we prevent this bypass. For VMs
users who have already followed the setup instructions, the following
command should (to be ran in dom0) will fix this issue:
qvm-prefs dz-dvm default_dispvm ''
