spapr: Add forgotten capability to migration stream

spapr: Add forgotten capability to migration stream

spapr machine capabilities are supposed to be sent in the migration stream
so that we can sanity check the source and destination have compatible
configuration.  Unfortunately, when we added the hpt-max-page-size
capability, we forgot to add it to the migration state.  This means that we
can generate spurious warnings when both ends are configured for large
pages, or potentially fail to warn if the source is configured for huge
pages, but the destination is not.

Fixes: 2309832afda "spapr: Maximum (HPT) pagesize property"
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>