qdev_try_create(): Assert that devices we put onto the system bus are SysBusDevices

If qdev_try_create() is passed NULL for the bus, it will automatically
put the newly created device onto the default system bus. However
if the device is not actually a SysBusDevice then this will result
in later crashes (for instance when running the monitor "info qtree"
command) because code reasonably assumes that all devices on the system
bus are system bus devices.

Generally the mistake is that the calling code should create the
object with object_new(TYPE_FOO) rather than qdev_create(NULL, TYPE_FOO);
see commit 6749695e for an example of fixing this bug.

Assert in qdev_try_create() if the device isn't suitable to put on
the system bus, so that this mistake results in failure earlier
and more reliably.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>