spapr: Don't migrate the hpt_maxpagesize cap to older machine types

Commit 0b8c89be7f7b added the hpt_maxpagesize capability to the migration
stream. This is okay for new machine types but it breaks backward migration
to older QEMUs, which don't expect the extra subsection.

Add a compatibility boolean flag to the sPAPR machine class and use it to
skip migration of the capability for machine types 4.0 and older. This
fixes migration to an older QEMU. Note that the destination will emit a
warning:

qemu-system-ppc64: warning: cap-hpt-max-page-size lower level (16) in incoming stream than on destination (24)

This is expected and harmless though. It is okay to migrate from a lower
HPT maximum page size (64k) to a greater one (16M).

Fixes: 0b8c89be7f7b "spapr: Add forgotten capability to migration stream"
Based-on: <20190522074016.10521-3-clg@kaod.org>
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155853262675.1158324.17301777846476373459.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>