hw/s390x: Add the possibility to specify the netboot image on the command line

The file name of the netboot binary is currently hard-coded to
"s390-netboot.img", without a possibility for the user to select
an alternative firmware image here. That's unfortunate, especially
since the basics are already there: The filename is a property of
the s390-ipl device. So we just have to add a check whether the user
already provided the property and only set the default if the string
is still empty. Now it is possible to select a different firmware
image with "-global s390-ipl.netboot_fw=/path/to/s390-netboot.img".
Signed-off-by: NThomas Huth <thuth@redhat.com>
Message-Id: <1519731154-3127-1-git-send-email-thuth@redhat.com>
Signed-off-by: NCornelia Huck <cohuck@redhat.com>

hw/s390x/s390-virtio-ccw.c

@@ -254,8 +254,10 @@ static void s390_init_ipl_dev(const char *kernel_filename,
     }
     qdev_prop_set_string(dev, "cmdline", kernel_cmdline);
     qdev_prop_set_string(dev, "firmware", firmware);
-    qdev_prop_set_string(dev, "netboot_fw", netboot_fw);
     qdev_prop_set_bit(dev, "enforce_bios", enforce_bios);
+    if (!strlen(object_property_get_str(new, "netboot_fw", &error_abort))) {
+        qdev_prop_set_string(dev, "netboot_fw", netboot_fw);
+    }
     object_property_add_child(qdev_get_machine(), TYPE_S390_IPL,
                               new, NULL);
     object_unref(new);