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>

hw/core/nmi.c

@@ -22,7 +22,6 @@
 #include "qemu/osdep.h"
 #include "hw/nmi.h"
 #include "qapi/error.h"
-#include "qapi/qmp/qerror.h"
 #include "qemu/module.h"
 #include "monitor/monitor.h"
 
@@ -70,7 +69,7 @@ void nmi_monitor_handle(int cpu_index, Error **errp)
     if (ns.handled) {
         error_propagate(errp, ns.err);
     } else {
-        error_setg(errp, QERR_UNSUPPORTED);
+        error_setg(errp, "machine does not provide NMIs");
     }
 }