1. A computer-implemented method for operating virtual resources, comprising: under control of one or more computer systems configured with executable instructions, providing virtual resources provisioned to use physical hardware resources of a virtual resource provider; providing a plurality of control planes within the virtual resource provider that perform management of at least some of the virtual resources, each of the control planes implemented by one or more of a plurality of control plane modules; establishing a custom virtual control plane that performs management of two or more of the virtual resources within the virtual resource provider, the custom virtual control plane being customizable through a control plane provisioning interface for selection of which control plane modules are associated with the virtual control plane; receiving at least a first resource configuration request from the custom virtual control plane and at least a second resource configuration request from at least one other of the plurality of control planes, the first and second resource configuration requests specifying competing configurations for a capacity of a resource, which is one of the physical hardware resources of the virtual resource provider; mediating the first and second resource configuration requests so as to maintain the resource within its capacity.