hw/pci-host/ppce500: Fix typo in vmstate definition

Fix a typo in the ppce500_pci vmstate definition which meant that
we were migrating the struct pci_inbound using the vmstate for
pci_outbound. Fortunately the two structures have exactly the same
format at the moment (four uint32_ts) so this was harmless, and
we can correcting the typo without a migration compatibility
break because the vmstate name doesn't go out on the wire.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>