qdev: relax bus type check in qdev_device_add() (v2)

We are currently checking for an exact type match.  Use QOM dynamic_cast to
check for a compatible type instead.

Cc: Konrad Frederic <fred.konrad@greensocs.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
v1 -> v2:
 - also add cast to qbus_find_recursive (Peter)
 - simplify by doing object_dynamic_cast instead of messing with classes
