qemu: Fix bug assuming usage of default UUID for certificate passphrase
    
    If an environment specific _tls_x509_cert_dir is provided, then
    do not VIR_STRDUP the defaultTLSx509secretUUID as that would be
    for the "default" environment and not the vnc, spice, chardev, or
    migrate environments. If the environment needs a secret to decode
    it's certificate, then it must provide the secret. If the secrets
    happen to be the same, then configuration would use the same UUID
    as the default (but we cannot assume that nor can we assume that
    the secret would be necessary).