virsh: Add/allow secret-uuid for pool-{define|create}-as
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1476775
    
    For the virsh pool-{define|create}-as command, let's allow using
    --secret-uuid on the command line as an alternative to --secret-usage
    (added for commit id '8932580'), but ensure that they are mutually
     exclusive.