Add a new qemu.conf variables to store the UUID for the secret that could
be used to present credentials to access the TLS chardev.  Since this will
be a server level and it's possible to use some sort of default, introduce
both the default and chardev logic at the same time making the setting of
the chardev check for it's own value, then if not present checking whether
the default value had been set.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>