hw/qdev: Don't crash if qdev_create(NULL, ...) fails

If an attempt to create a qdev device on the default sysbus (by passing
NULL as the bus to qdev_create) fails, print a useful error message
rather than crashing trying to dereference a NULL pointer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
