1. A computer-implemented method for placing virtual machines on physical host computer systems, the method comprising: creating at least one host group comprising a plurality of physical host computer systems; allowing a system administrator to set a first set of policies and a second set of policies for the at least one host group that respectively indicates how to place virtual machines on the plurality of physical host computer systems; wherein the first set of policies are initial placement policies that indicate how to initially place virtual machines on the plurality of physical host computer systems; wherein the second set of policies are optimization placement policies that indicate how to place virtual machines on the plurality of physical host computer systems when optimizing the physical host computer systems; wherein the initial placement policies are simple policies for quickly determining an initial host for a virtual machine that are distinct from the more detailed optimization policies for migrating virtual machines to optimize placement; placing virtual machines on the physical host computer systems of the at least one host group according to the initial placement policies; allowing the system administrator to set a first threshold for the initial placement policies and a second threshold for the optimization placement policies; monitoring performance of the physical host computer systems of the at least one host group; and optimizing placement of the virtual machines on the physical host computer systems of the at least one host group when the performance meets the second threshold of the optimization placement policies by migrating at least one of the virtual machines to a different one of the physical host computer systems.