1. A computing method, comprising: specifying a virtual computer system comprising at least one virtual or physical compute node, which produces data packets having respective source attributes; defining at least one Virtual Input-Output Connection (VIOC) that is uniquely associated with the values of the source attributes; defining a policy specifying an operation to be performed with regard to the VIOC; implementing the virtual computer system on a physical computer system that includes at least one physical packet switching element; and configuring the physical packet switching element to identify the data packets whose source attributes have the values that are associated with the VIOC and to perform the operation on the identified data packets, so as to enforce the policy on the VIOC, wherein configuring the physical packet switching element comprises detecting a change in one of the physical computer system and the virtual computer system, and re-configuring the physical packet switching element responsively to the detected change, so as to cause the physical packet switching element to continue enforcing the policy, wherein the at least one physical packet switching element comprises first and second physical packet switching elements, wherein configuring the physical packet switching element comprises initially configuring the first physical packet switching element to enforce the policy, and wherein re-configuring the physical packet switching element comprises, responsively to detecting the change, configuring the second physical packet switching element to enforce the policy.