qom: use object_new_with_class when possible

qom: use object_new_with_class when possible

A small optimization/code simplification, that also makes it clear that
we won't look for a type in a not-loaded-yet module---the module will
have been loaded by a call to module_object_class_by_name(), if present.

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