qapi: Factor out QAPISchemaObjectType.check_clash()

Consolidate two common sequences of clash detection into a
new QAPISchemaObjectType.check_clash() helper method.

No change to generated code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1447836791-369-13-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
