qom: let object_new use a module if the type is not present

qom: let object_new use a module if the type is not present

object_initialize() can use modules (it was added there because
virtio-gpu-device is a child device of virtio-gpu-pci; commit
64f7aece8ea, "object_initialize: try module load", 2020-09-15).
object_new() cannot; make things consistent.

qdev_new() is now just a simple wrapper that returns DeviceState.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>