For union types, the tag member is known only after .check().
We used to code this in a simple way: QAPISchemaVariants attribute
.tag_member was None for union types until .check().
Since this complicated typing, recent commit "qapi/schema: fix typing
for QAPISchemaVariants.tag_member" hid it behind a property.
The previous commit lets us treat .tag_member just like the other
attributes that become known only in .check(): declare, but don't
initialize it in .__init__().
Signed-off-by: Markus Armbruster <armbru@redhat.com>
285a8f209a