qapi/machine: Clarify query-uuid value when none has been specified

qapi/machine: Clarify query-uuid value when none has been specified

When no UUID has been specified, query-uuid returns

    {"UUID": "00000000-0000-0000-0000-000000000000"}

The doc comment calls this "a null UUID", which I find less than
clear.  RFC 9562 calls it "the nil UUID (all zeroes)", so use that
instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240711112228.2140606-4-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
[Wording improved, commit message adjusted]
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>