1. A computer-implemented method for placing virtual machines on physical host computer systems, the method comprising: creating a virtual machine group comprising a plurality of virtual machines; assigning a collocation rule for the virtual machine group; setting a plurality of situational enforcement policies for a plurality of situations that indicate how to place the plurality of virtual machines on a physical host computer system for the plurality of situations; placing the plurality of virtual machines on the physical host computer system based on the collocation rule and the plurality of situational enforcement policies.