conf: Add new secret type "tls"

conf: Add new secret type "tls"

Add a new secret usage type known as "tls" - it will handle adding the
secret objects for various TLS objects that need to provide some sort
of passphrase in order to access the credentials.

The format is:

   <secret ephemeral='no' private='no'>
     <description>Sample TLS secret</description>
     <usage type='tls'>
       <name>mumblyfratz</name>
     </usage>
</secret>

Once defined and a passphrase set, future patches will allow the UUID
to be set in the qemu.conf file and thus used as a secret for various
TLS options such as a chardev serial TCP connection, a NBD client/server
connection, and migration.

Signed-off-by: John Ferlan <jferlan@redhat.com>