Add missing "%s" format to const error message in RBD storage driver

When passing a const message string to the error reporting APIs
RBD forgot to use "%s" to avoid GCC format string warnings
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/storage/storage_backend_rbd.c

@@ -145,7 +145,7 @@ static int virStorageBackendRBDOpenRADOSConn(virStorageBackendRBDStatePtr *ptr,
                               pool->def->source.hosts[i].name,
                               pool->def->source.hosts[i].port);
         } else {
-            virStorageReportError(VIR_ERR_INTERNAL_ERROR,
+            virStorageReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                                   _("received malformed monitor, check the XML definition"));
         }
     }