spapr/htab: fix savevm

Commit 3a38429 ("spapr: Add a "no HPT" encoding to HTAB migration stream")
allows to migrate an empty HPT, but doesn't mark correctly the
end of the migration stream.

The end condition (value returned by htab_save_iterate())
should be 1, whereas in 3a38429 it returns 0.

The problem can be reproduced with QEMU monitor command "savevm":
the command never stops and the disk image grows without limit.

Fixes: 3a38429748aa4f74abaecf16c4c087e8a325e12a
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
