qapi: Rename anonymous union type in test Reduce churn in the future patch that replaces anonymous unions with a new metatype 'alternate' by changing 'AnonUnion' to 'Alternate'. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: Markus Armbruster <armbru@redhat.com>