watchdog/wdt_diag288: Mark diag288 watchdog as non-hotpluggable

QEMU currently aborts when the user tries to hot-unplug a diag288
device:

$ qemu-system-s390x -nographic -nodefaults -S -monitor stdio
QEMU 2.9.92 monitor - type 'help' for more information
(qemu) device_add diag288,id=x
(qemu) device_del x
**
ERROR:qemu/qdev-monitor.c:872:qdev_unplug: assertion failed: (hotplug_ctrl)
Aborted (core dumped)

The device is not designed as hot-pluggable (it should only be used
via the "-watchdog" parameter), so let's simply remove the possibility
to hotplug it to prevent that users can run into this ugly situation.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1502892528-22618-1-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>