vl.c: select_machine(): add selected machine type to error message

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

system/vl.c

@@ -1674,7 +1674,7 @@ static MachineClass *select_machine(QDict *qdict, Error **errp)
         machine_class = find_machine(machine_type, machines);
         qdict_del(qdict, "type");
         if (!machine_class) {
-            error_setg(errp, "unsupported machine type");
+            error_setg(errp, "unsupported machine type: \"%s\"", optarg);
         }
     } else {
         machine_class = find_default_machine(machines);