py3 dnssec: convert hexlify to str
    
    hexlify returns bytes and needs to be casted to string before
    printing it out.
    
    Related: https://pagure.io/freeipa/issue/4985
    
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>