virtio-serial-bus: use virtio wrappers to access headers

virtio-serial-bus: use virtio wrappers to access headers

We also fix max_nr_ports at reset time as the device endianness may have
changed.

Signed-off-by: Rusty Russell <[email protected]>
Reviewed-by: Anthony Liguori <[email protected]>
[ pass VirtIODevice * to memory accessors,
  fix max_nr_ports at reset time,
  Greg Kurz <[email protected]> ]
Signed-off-by: Greg Kurz <[email protected]>
Reviewed-by: Alexander Graf <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>