spapr: Fix 2.7<->2.8 migration of PCI host bridge

spapr: Fix 2.7<->2.8 migration of PCI host bridge

daa2369 "spapr_pci: Add a 64-bit MMIO window" subtly broke migration
from qemu-2.7 to the current version.  It split the device's MMIO
window into two pieces for 32-bit and 64-bit MMIO.

The patch included backwards compatibility code to convert the old
property into the new format.  However, the property value was also
transferred in the migration stream and compared with a (probably
unwise) VMSTATE_EQUAL.  So, the "raw" value from 2.7 is compared to
the new style converted value from (pre-)2.8 giving a mismatch and
migration failure.

Along with the actual field that caused the breakage, there are
several other ill-advised VMSTATE_EQUAL()s.  To fix forwards
migration, we read the values in the stream into scratch variables and
ignore them, instead of comparing for equality.  To fix backwards
migration, we populate those scratch variables in pre_save() with
adjusted values to match the old behaviour.

To permit the eventual possibility of removing this cruft from the
stream, we only include these compatibility fields if a new
'pre-2.8-migration' property is set.  We clear it on the pseries-2.8
machine type, which obviously can't be migrated backwards, but set it
on earlier machine type versions.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>