PPC: Fix crash on spapr_tce_table_finalize()

spapr_tce_table_finalize() can SEGV if the object was not previously
realized.  In particular this can be triggered by running
         qemu-system-ppc -device spapr-tce-table,?

The basic problem is that we have mismatched initialization versus
finalization: spapr_tce_table_finalize() is attempting to undo things that
are done in spapr_tce_table_realize(), not an instance_init function.

Therefore, replace spapr_tce_table_finalize() with
spapr_tce_table_unrealize().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Cc: qemu-stable@nongnu.org
Signed-off-by: Alexander Graf <agraf@suse.de>