s390x/ipl: The s390-ipl device is not hot-pluggable

The s390-ipl device can not be created by the user, since it is meant only
to  be instantiated once internally to load the ROMs and kernel. If the user
tries to do a "device_add s390-ipl" via the monitor later, QEMU aborts with
a "ROM images must be loaded at startup" error message.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1502861458-30270-1-git-send-email-thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
(cherry picked from commit 0d4fa499)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>