acpi_piix4: fix segfault migrating from 1.2

acpi_piix4: fix segfault migrating from 1.2

b0b873a07872f7ab7f66f259c73fb9dd42aa66a9 bumped the vmstate version and
introduced an old-style load function to handle migration from prior
(<= 1.2) versions.

The load function passes the top-level PIIX4PMState pointer to
vmstate_load_state() to handle nested structs for APMState and
pci_status, which leads to corruption of the top-level PIIX4PMState,
since pointers to the nested structs are expected.

A segfault can be fairly reliably triggered by migrating from 1.2 and
issuing a reset, which will trigger a number of QOM operations which
rely on the now corrupted ObjectClass/Object members.

Fix this by passing in the expected pointers for vmstate_load_state().

Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>