hw/net/ne2000-isa: Register vmstate struct

The ne2000-isa device defines a VMState struct for migration, but
we forgot to actually register it. Correct this deficiency by
setting dc->vmsd.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/net/ne2000-isa.c

@@ -98,6 +98,7 @@ static void isa_ne2000_class_initfn(ObjectClass *klass, void *data)
 
     dc->realize = isa_ne2000_realizefn;
     dc->props = ne2000_isa_properties;
+    dc->vmsd = &vmstate_isa_ne2000;
     set_bit(DEVICE_CATEGORY_NETWORK, dc->categories);
 }