This is to allow permissions to be set on some devices where the user needs
less restrictive permissions. /etc/udev/rules.d/99-qubes-misc.rules changes
a few xen devices to allow the users in the qubes group access
Generate user-groups via -U instead of explicit via groupadd. This also
fix the problem that the tinyproxy group were not gererated as
"system"-group.
Also suppress unneeded output of the existence test.
