Fix VisualDataModel auto test failure.

Set the DynamicMetaObject flag when constructing an extended type.
Fixes failure introduced in 038619bf.

Change-Id: Idbbbec8f9ec3792b2765639781dba1696fce0a49
Reviewed-on: http://codereview.qt.nokia.com/2927


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Andrew den Exter <andrew.den-exter@nokia.com>