storage: Fix logical pool fmt type
    
    According to our documentation logical pool supports formats 'auto' and
    'lvm2'. However, in storage_conf.c we previously defined storage pool
    formats: unknown, lvm2. Due to backward compatibility reasons
    we must continue refer to pool format type 'unknown' instead of 'auto'.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1123767