hw/s390x: Mark the "sclpquiesce" device with user_creatable = false

The "sclpquiesce" device is just an internal device that should not be
created by the user directly. Though it currently does not seem to cause
any obvious trouble when the user instantiates an additional device, let's
better mark it with user_creatable = false to avoid unexpected behavior,
e.g. because the quiesce notifier gets registered multiple times.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1507193105-15627-1-git-send-email-thuth@redhat.com>
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
