machine: Polish -machine xxx,help

The "-machine xxx,help" prints kernel-irqchip possible values as
"OnOffSplit", this adds separators to the printed line.

Also, since only lower case letters are specified in qapi/common.json,
this changes the letter cases too.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/core/machine.c

@@ -520,7 +520,7 @@ static void machine_class_init(ObjectClass *oc, void *data)
     object_class_property_set_description(oc, "accel",
         "Accelerator list", &error_abort);
 
-    object_class_property_add(oc, "kernel-irqchip", "OnOffSplit",
+    object_class_property_add(oc, "kernel-irqchip", "on|off|split",
         NULL, machine_set_kernel_irqchip,
         NULL, NULL, &error_abort);
     object_class_property_set_description(oc, "kernel-irqchip",