qapi: Simplify QAPISchemaObjectTypeVariants.check()

Reduce the ugly flat union / simple union conditional by doing just
the essential work here, namely setting self.tag_member.
Move the rest to callers.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1446559499-26984-7-git-send-email-armbru@redhat.com>
[rebase to earlier changes that moved tag_member.check() of
alternate types, and tweak commit title and wording]
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-11-git-send-email-eblake@redhat.com>
