Make sure .qch file is opened read-only

So far qsqlite tried to open the file first read/write, and falling back
to read access only if this fails. This has resulted in excessively long
loading times e.g. on Windows XP.

Task-number: QTCREATORBUG-10697

(cherry-picked from commit 0bf84aa7873edd7bb411fabb94481b03d1501c68 in
qttools.git)

Change-Id: I94be15e2d7c5845bcadf6b32e54462a7c1bcb98a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>