'Preparation live migration.
:param block_device_info:
It must be the result of _get_instance_volume_bdms()
at compute manager.
:returns: a XenapiLiveMigrateData object'

def pre_live_migration(self, context, instance, block_device_info, network_info, disk_info, migrate_data):

    return self._vmops.pre_live_migration(context, instance, block_device_info, network_info, disk_info, migrate_data)
