qapi: Hide tag_name data member of variants

qapi: Hide tag_name data member of variants

Clean up the only remaining external use of the tag_name field of
QAPISchemaObjectTypeVariants, by explicitly listing the generated
'type' tag for all variants in the testsuite (you can still tell
simple unions by the -wrapper types).  Then we can mark the
tag_name field as private by adding a leading underscore to prevent
any further use.

Signed-off-by: Eric Blake <[email protected]>
Message-Id: <1468468228[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>