qemu: Fix bug assuming usage of default UUID for certificate passphrase

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).