hw/core: Improve error message when machine doesn't provide NMIs

hw/core: Improve error message when machine doesn't provide NMIs

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_UNSUPPORTED, and improve the rather vague
error message

    (qemu) nmi
    Error: this feature or command is not currently supported

to

    Error: machine does not provide NMIs

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-5-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>