1. An apparatus comprising: at least one processor; a memory coupled to the at least one processor; a first host group comprising a plurality of physical host computer systems; an initial placement policy for the first host group that indicates how to initially place virtual machines on the plurality of physical host computer systems; an optimization policy for the first host group that indicates how to place virtual machines on the plurality of physical host computer systems to optimize performance of the physical host computer systems in the first host group; a scheduler mechanism residing in the memory and executed by the at least one processor that places virtual machines on physical host computer systems of the first host group according to the initial placement policy and optimizes placement of the physical host computer systems of the host group according to the optimization policy by changing the placement of at least one of the virtual machines on the physical host computer systems.