authz: add QAuthZSimple object type for easy whitelist auth checks

authz: add QAuthZSimple object type for easy whitelist auth checks

In many cases a single VM will just need to whitelist a single identity
as the allowed user of network services. This is especially the case for
TLS live migration (optionally with NBD storage) where we just need to
whitelist the x509 certificate distinguished name of the source QEMU
host.

Via QMP this can be configured with:

  {
    "execute": "object-add",
    "arguments": {
      "qom-type": "authz-simple",
      "id": "authz0",
      "props": {
        "identity": "fred"
      }
    }
  }

Or via the command line

  -object authz-simple,id=authz0,identity=fred

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>