vnet: print Success for API errno 0 instead of UNKNOWN

Type: improvement
Change-Id: I8c5bfc82c1db1213eb43072853f8500ce1d2ee92
Signed-off-by: Klement Sekera <klement.sekera@gmail.com>

src/vnet/error.c

@@ -37,6 +37,10 @@ u8 *
 format_vnet_api_errno (u8 *s, va_list *args)
 {
   vnet_api_error_t api_error = va_arg (*args, vnet_api_error_t);
+
+  if (0 == api_error)
+    return format (s, "Success");
+
 #ifdef _
 #undef _
 #endif