qdev: add a maximum device allowed field for the bus.

Add a max_dev field to BusClass to specify the maximum amount of devices allowed
on the bus (has no effect if max_dev=0)

Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>