proxy_internal.c: mark "request" parameter as nonnull

* src/xen/proxy_internal.c (xenProxyCommand): Mark "request"
as an always-non-NULL parameter.

src/xen/proxy_internal.c

@@ -340,7 +340,7 @@ xenProxyClose(virConnectPtr conn)
     return 0;
 }
 
-static int
+static int ATTRIBUTE_NONNULL(2)
 xenProxyCommand(virConnectPtr conn, virProxyPacketPtr request,
                 virProxyFullPacketPtr answer, int quiet) {
     static int serial = 0;