conf: Use the correct limit for the number of PHBs
    
    I mistakenly thought pSeries guests supported 32 PHBs,
    but it turns out they only support 31. Validate the
    target index accordingly.
    
    Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1479647
    
    Signed-off-by: Andrea Bolognani <abologna@redhat.com>