1. A computer-implemented method comprising: maintaining, by a host controller, a hierarchy of labels, each of the labels representing a distinct virtual machine parameter; allowing a user to specify labels for a plurality of virtual machines hosted by one or more servers coupled to the host controller; storing the specified labels in association with respective virtual machines in a database that reflects relationships between the labels according to the hierarchy; and facilitating, by the host controller, management of the plurality of virtual machines using the labels.