spapr: migration support for CAS-negotiated option vectors

With the additional of the OV5_HP_EVT option vector, we now have
certain functionality (namely, memory unplug) that checks at run-time
for whether or not the guest negotiated the option via CAS. Because
we don't currently migrate these negotiated values, we are unable
to unplug memory from a guest after it's been migrated until after
the guest is rebooted and CAS-negotiation is repeated.

This patch fixes this by adding CAS-negotiated options to the
migration stream. We do this using a subsection, since the
negotiated value of OV5_HP_EVT is the only option currently needed
to maintain proper functionality for a running guest.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>