security: improve security driver error message

Currently, when security driver is not available users are informed that
it wasn't found which can be confusing.
1. Update error message
2. Add comment to domain doc
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by Sebastian Mitterle <smitterl@redhat.com>

docs/formatdomain.html.in

@@ -9011,7 +9011,8 @@ qemu-kvm -net nic,model=? /dev/null
       </dd>
       <dt><code>model</code></dt>
       <dd>A valid security model name, matching the currently
-        activated security model
+      activated security model. Model <code>dac</code> is not available
+      when guest is run by unprivileged user.
       </dd>
       <dt><code>relabel</code></dt>
       <dd>Either <code>yes</code> or <code>no</code>. This must always

src/security/security_manager.c

@@ -769,7 +769,7 @@ static int virSecurityManagerCheckModel(virSecurityManagerPtr mgr,
     }
 
     virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                   _("Unable to find security driver for model %s"),
+                   _("Security driver model '%s' is not available"),
                    secmodel);
  cleanup:
     VIR_FREE(sec_managers);