hw/core: Improve the query-hotpluggable-cpus error message

hw/core: Improve the query-hotpluggable-cpus error message

The QERR_ macros are leftovers from the days of "rich" error objects.
We've been trying to reduce their remaining use.

Get rid of a use of QERR_FEATURE_DISABLED, and improve the slightly
awkward error message

    (qemu) info hotpluggable-cpus
    Error: The feature 'query-hotpluggable-cpus' is not enabled

to

    Error: machine does not support hot-plugging CPUs

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-11-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>