1. A storage management computer for managing offloading of storage workload between a storage controller of a storage system and one or more host computers, the storage management computer comprising: a processor; a controller operable to request a virtual machine management computer to register the storage controller as a host computer, and to send, to the virtual machine management computer, storage processes information of storage processes in the storage system which can be offloaded as virtual machines in order for the virtual machine management computer to register the storage processes as virtual machines; wherein the controller is operable to request the virtual machine management computer to add the storage controller registered as a host computer into an optimization group of host computers, and to register one or more allocation rules for allocating virtual machines among the host computers in the optimization group, including an allocation rule that prohibits virtual machines except the storage processes that are registered as virtual machines to be migrated to the storage controller registered as a host computer; wherein migration of virtual machines between the host computers in the optimization group is performed according to the one or more allocation rules; and wherein, upon receiving a virtual machine migration request from a destination host computer to migrate one or more storage processes registered as virtual machines from the storage controller registered as a host computer to the destination host computer, the controller is operable to migrate the one or more storage processes registered as virtual machines, from the storage controller registered as a host computer to the destination host computer, via an offload target port on the storage controller.