tests/qapi-schema: Document alternate's enum lacks visit function

We generate a declaration, but no definition.

The QMP schema has two: Qcow2OverlapChecks and BlockdevRef.  Neither
visit_type_Qcow2OverlapChecksKind() nor visit_type_BlockdevRefKind()
is actually used.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>