As pointed out in 13b939c7

 the approach
qdevicediscovery_p.h takes is just wrong. The defines it relies on will
often be missing when the header is included from random places in qtbase.
This results in different class layouts. It was working only because the
interface of the class is very limited and the public part was matching
regardless of having the macro defined.

This is now corrected by introducing subclasses and a common, non-variated
base class. QDEVICEDISCOVERY_UDEV is removed completely.

Change-Id: I9c83b5b041440a3a6ea3a604eee4a325d4d74439
Reviewed-by: Louai Al-Khanji <louai.al-khanji@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>