qapi/schema: fix typing for QAPISchemaVariants.tag_member

qapi/schema: fix typing for QAPISchemaVariants.tag_member

There are two related changes here:

(1) We need to perform type narrowing for resolving the type of
    tag_member during check(), and

(2) tag_member is a delayed initialization field, but we can hide it
    behind a property that raises an Exception if it's called too
    early. This simplifies the typing in quite a few places and avoids
    needing to assert that the "tag_member is not None" at a dozen
    callsites, which can be confusing and suggest the wrong thing to a
    drive-by contributor.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240315152301.3621858-18-armbru@redhat.com>