docs: correct values for old VMDP unplug

docs: correct values for old VMDP unplug

Fix commit f6d4cf5 ("docs: document old SUSE/Novell unplug for HVM").
The values which VMDP used to control either NIC or disk are flipped.
What the code does is:

     case 8:
        if (val == 1 ) {
                ide_unplug_harddisks();
        } else if (val == 2) {
                pci_unplug_netifs();
                net_tap_shutdown_all();
        }
        break;

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>