Make use of the internal space for enum in QVariant

Enums are movable even if they have not been tagged as such, and
therefore can safely make use of the internal storage.

Change-Id: I3bec8556bfeb4a07cb30d918f8d1dd2565d6d94a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>