Add more types to QVariant->VARIANT conversion.

Introduce a macro for POD type conversion and add char, short
and float.

Remove the type coercion branch which is dead in Qt 5 since
QVariant::nameToType() always returns a valid type for char,
short and float.

Add autotest.

Task-number: QTBUG-45057
Change-Id: If3684d229a653c51b67015a7ef0c4631ead64a94
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>