qom: add object_new_with_class

qom: add object_new_with_class

Similar to CPU and machine classes, "-accel" class names are mangled,
so we have to first get a class via accel_find and then instantiate it.
Provide a new function to instantiate a class without going through
object_class_get_name, and use it for CPUs and machines already.

Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>