QtIFW

Make sure we close the file once we read the content.

Fixes exception thrown on windows in case we can't read the
content or got an repository update. This can happen since
the finish signal is emitted synchronously and the following
code is trying to delete the still opened updates xml file.

Change-Id: I32b252e372ed11c05b04e0d2736400527bdb9a59
Reviewed-by: Niels Weber <niels.weber@digia.com>