We set machine_class to the default first, and update it to the real
one later.  Any use of machine_class in between is almost certainly
wrong (there are no such uses right now).  Set it once and for all
instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>