The calculus of virtually timed ambients models timing aspects of resource management for virtual machines. With nested virtualization, virtual machines compete with other processes for the resources of their host environment. Resource provisioning in virtually timed ambients can be formalized by extending the capabilities of mobile ambients to model the dynamic creation, migration, and destruction of virtual machines. This paper introduces a logic to define modal contracts regarding resource management for virtually timed ambients. Service-level agreements are contracts between a service provider and a client, specifying properties that the service should fulfill with respect to quality of service (QoS). The proposed modal logic supports QoS statements about the resource consumption and nesting structure of a system during the timed reduction of its processes. Besides a formal definition of the logic, the paper provides a corresponding model checking algorithm and its prototype implementation in rewriting logic.