qapi: Tweak code to match docs/devel/qapi-code-gen.txt

qapi: Tweak code to match docs/devel/qapi-code-gen.txt

The previous commit made qapi-code-gen.txt define "(top-level)
expression" as either "directive" or "definition".  The code still
uses "expression" when it really means "definition".  Tidy up.

The previous commit made qapi-code-gen.txt use "object" rather than
"dictionary".  The code still uses "dictionary".  Tidy up.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190913201349.24332-17-armbru@redhat.com>