Throw an assert when using an unsupported variant type

with QgsXmlUtils::writeVariant

These were previously written as strings yet could not be
restored by QgsXmlUtils::readVariant. Better to throw
an assert so that it's clear that these types are unsupported
and that support needs to be added.

Author: nyalldawson

src/core/qgsxmlutils.cpp

@@ -154,8 +154,7 @@ QDomElement QgsXmlUtils::writeVariant( const QVariant &value, QDomDocument &doc
       break;
 
     default:
-      element.setAttribute( QStringLiteral( "type" ), QStringLiteral( "Unknown" ) );
-      element.setAttribute( QStringLiteral( "value" ), value.toString() );
+      Q_ASSERT_X( false, "QgsXmlUtils::writeVariant", "unsupported variant type" );
       break;
   }