Revert "spapr: Fix migration of PCI host bridges from qemu-2.7"

Revert "spapr: Fix migration of PCI host bridges from qemu-2.7"

This reverts commit 9b54ca0ba781012eeea4237b7c4832ba2ea81d89.

The commit above corrected a migration breakage between qemu-2.7 and
qemu-2.8.  However it did so by advancing the migration version for
the PCI host bridge, which obviously breaks migration backwards to
earlier qemu versions.

Although it's not totally essential, we'd like to maintain the
possibility for backwards migration, so revert the change in
preparation for a better fix.

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