tools/libxl: Document where the magic MAC numbers come from

Matches the comment in the xl-network-configuration manpage.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>

tools/libs/light/libxl_nic.c

@@ -73,6 +73,7 @@ static int libxl__device_nic_setdefault(libxl__gc *gc, uint32_t domid,
         libxl_uuid_generate(&uuid);
         r = libxl_uuid_bytearray(&uuid);
 
+        /* Generate a random MAC address, with Xen's OUI (00:16:3e) */
         nic->mac[0] = 0x00;
         nic->mac[1] = 0x16;
         nic->mac[2] = 0x3e;