virtio_config: disallow native type fields (again)

_Generic version allowed __uXX types but that is no longer necessary:

Transitional devices should all use __virtioXX types (and __leXX for
fields not present in the legacy devices).
Modern ones should use __leXX.
_uXX type would be a bug.
Let's prevent that.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>