QNAM: Fix CPU load for limited upload QIODevice

This fixes high CPU load for upload devices that don't generate
a constant stream of data. Their readData() function was called all the
time without returning actual data.

This was noticed when implementing an upload device that emits data in
a limited way for bandwidth limiting.

[ChangeLog][QtNetwork][QNetworkAccessManager] Fixed high CPU load when handling
POST/upload QIODevice that generates data on readyRead().

Change-Id: Iefbcb1a21d8aedef1eb11761232dd16a049018dc
Reviewed-by: Richard J. Moore <rich@kde.org>