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>