libxc: increase maximum migration stream record length
Today the maximum record lenth in a migration stream is 8MB. This
limits the size of a PV domain to a little bit less than 1TB in the
migration case, as the P2M frame list will exceed 8MB in this case.
Raising the record size limit by a factor of 16 allows for domain
sizes of nearly 16TB to be migrated. This ought to be enough.