hw/display/macfb: Classify the "nubus-macfb" as display device

The "nubus-macfb" currently shows up as uncategorized device in
the output of "-device help". Put it into the display category
to fix this ugliness.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210531073255.46286-1-thuth@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

hw/display/macfb.c

@@ -450,6 +450,7 @@ static void macfb_nubus_class_init(ObjectClass *klass, void *data)
     dc->desc = "Nubus Macintosh framebuffer";
     dc->reset = macfb_nubus_reset;
     dc->vmsd = &vmstate_macfb;
+    set_bit(DEVICE_CATEGORY_DISPLAY, dc->categories);
     device_class_set_props(dc, macfb_nubus_properties);
 }