secret: rework handling of private secrets

secret: rework handling of private secrets

A secret can be marked with the "private" attribute. The intent was that
it is not possible for any libvirt client to be able to read the secret
value, it would only be accesible from within libvirtd. eg the QEMU
driver can read the value to launch a guest.

With the modular daemons, the QEMU, storage and secret drivers are all
running in separate daemons. The QEMU and storage drivers thus appear to
be normal libvirt client's from the POV of the secret driver, and thus
they are not able to read a private secret. This is unhelpful.

With the previous patches that introduced a "system token" to the
identity object, we can now distinguish APIs invoked by libvirt daemons
from those invoked by client applications.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>