QtIFW

Fix install fail if there is huge amount of downloadable packages

In some machines, installer does not manage to download meta files if
there are huge amount of downloadable packages. It seems that
QtConcurrent is not always able to handle so many download tasks.
Divided the downloadable tasks into size of 1000 to keep the
QtConcurrent running.

Change-Id: I3ba110e2cc565251e540349a6b073245280a6fbe
Reviewed-by: Ville Voutilainen <ville.voutilainen@qt.io>