spapr_iommu: Migrate full state

spapr_iommu: Migrate full state

The source guest could have reallocated the default TCE table and
migrate bigger/smaller table. This adds reallocation in post_load()
if the default table size is different on source and destination.

This adds @bus_offset, @page_shift to the migration stream as
a subsection so when DDW is added, migration to older machines will
still be possible. As @bus_offset and @page_shift are not used yet,
this makes no change in behavior.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>