hw/ppc/spapr_iommu: Fix crash when removing the "spapr-tce-table" device

QEMU currently aborts unexpectedly when the user tries to add and
remove a "spapr-tce-table" device:

$ qemu-system-ppc64 -nographic -S -nodefaults -monitor stdio
QEMU 2.9.92 monitor - type 'help' for more information
(qemu) device_add spapr-tce-table,id=x
(qemu) device_del x
**
ERROR:qemu/qdev-monitor.c:872:qdev_unplug: assertion failed: (hotplug_ctrl)
Aborted (core dumped)

The device should not be accessable for the users at all, it's just
used internally, so mark it with user_creatable = false.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
