This is a follow-up of a860eddbbd. Guided
installation tests are now run from an ISO image. Hence the main block device
is vda and not vdb anymore.
* gnu/tests/install.scm (installation-target-os-for-gui-tests): Use
%minimal-os-on-vda instead of %minimal-os.
(%minimal-os-on-vda): Make sure that it replicates the config of %minimal-os.