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: NMarkus 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: NEric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-11-git-send-email-eblake@redhat.com>

scripts/qapi.py

@@ -988,9 +988,10 @@ class QAPISchemaObjectType(QAPISchemaType):
         for m in self.local_members:
             m.check(schema)
             m.check_clash(seen)
+        self.members = seen.values()
         if self.variants:
             self.variants.check(schema, seen)
-        self.members = seen.values()
+            assert self.variants.tag_member in self.members
 
     def is_implicit(self):
         # See QAPISchema._make_implicit_object_type()
@@ -1050,10 +1051,8 @@ class QAPISchemaObjectTypeVariants(object):
         self.variants = variants
 
     def check(self, schema, seen):
-        if self.tag_name:    # flat union
+        if not self.tag_member:    # flat union
             self.tag_member = seen[self.tag_name]
-        if seen:
-            assert self.tag_member in seen.itervalues()
         assert isinstance(self.tag_member.type, QAPISchemaEnumType)
         for v in self.variants:
             v.check(schema, self.tag_member.type)