hw/ppc/spapr: fix spapr->kvm_type leak

Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Alexander Graf <agraf@suse.de>
Cc: qemu-ppc@nongnu.org
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
[fixed return type of spapr_machine_finalizefn()]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
