qga/qapi-schema.json: Add some headings

Add some section headings to the QGA json; this is purely so that we
have some H1 headings, as otherwise each command ends up being
visible in the interop/ manual's table of contents.  In an ideal
world there might be a proper 'Introduction' section the way there is
in qapi/qapi-schema.json.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-12-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>