QtIFW

Fix "Too many open files" error.

The current code did create all files in advance, exceeding the
open file limit on big downloads. Now we create the file once we
write to it.

Task-number: QTIFW-662
Change-Id: I9fe019e08342cbfb14bf564ad00b045cc03b6661
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>