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.