KArchive/KCompressionDevice regression with Qt >= 5.7

[Albert Astals Cid]

As shown by the tests i've just commited

https://build.kde.org/job/karchive%20master%20kf5-qt5/PLATFORM=Linux,compiler=gcc/142/console
(ignore the bit in that page it says it is using 5.6, it is not).

KArchive broke with Qt 5.7 (they rewrote some parts if QIODevice) 

Anyone that knows the code around? Is it Qt being buggy or new Qt paths exercising buggy code in KArchive?

I had a look yesterday at 2am but between being late and being a bit sick-ish didn't get much out of it.

Cheers,
  Albert