Update ch.15 - MTV performance recommendation

Doc: https://docs.redhat.com/en/documentation/migration_toolkit_for_virtualization/2.8/html/installing_and_using_the_migration_toolkit_for_virtualization/mtv-performance-recommendation_mtv#mtv-performance-recommendation_mtv

pls add to section 15.12, 'Maximum value tested for warm migration': (like in section 15.10)
Maximum single disk size migrated: 6TB disk, which contained 3TB of data