spec: create 'qubes' group is not exists

This group can be created also by qubes-core-dom0 package, but add
relevant code also here to simplify dependencies.

rpm_spec/core-dom0-linux.spec

@@ -143,6 +143,11 @@ done
 ### Documentation
 (cd doc; make DESTDIR=$RPM_BUILD_ROOT install)
 
+%pre
+if ! grep -q ^qubes: /etc/group ; then
+		groupadd qubes
+fi
+
 %post
 
 for i in /usr/share/qubes/icons/*.png ; do