spapr: don't migrate "spapr_option_vector_ov5_cas" to pre 2.8 machines

spapr: don't migrate "spapr_option_vector_ov5_cas" to pre 2.8 machines

a324d6f16697 "spapr: Support ibm,dynamic-memory-v2 property" added
a new feature in the set of CAS-negotiatable options. This causes
the CAS-negotiated options subsection to be migrated, even for old
machine types that don't know about it, and breaks backward migration
to QEMU 2.7 and older versions:

qemu-system-ppc64: error while loading state for instance 0x0 of device
 'spapr'
qemu-system-ppc64: load of migration failed: No such file or directory

Since this feature only affects boot time behaviour, it should be
filtered out when we decide to migrate CAS-negotiated options, like
we already do with OV5_FORM1_AFFINITY and OV5_DRCONF_MEMORY.

Signed-off-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>