QQmlImport: Don't try use a dangling pointer.

toUtf8 would return a temporary, and constData would hold a pointer inside that
temporary. This isn't even remotely safe. Move the pointer use down to the
initializeEngine call so it is kept around long enough for us to do our stuff.

This is a backport of cf51cdb8fb002ae3602a4c886e7c67913d77373a.

Task-number: QTBUG-35355
Task-number: QTBUG-35343
Change-Id: I338ad7f4d4137445ed9a311a293ea82bf023aafd
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>