Algorithm-chaining across cards.

It is possible that an OCF consumer needs to chain together a number of cryptographic algorithms, but no hardware producer implements all these. Currently, this would cause the session to be established on the software pseudo-driver (which implements all algorithms). However, by maintaining session information inside the OCF, it is possible to create ``virtual sessions'' across multiple (hardware and software) producers. In this case, the OCF will issue multiple sequential requests to the various producers, invoking the consumer-specified callback routine at the end. We have a prototype of this, but we need to further evaluate the performance implications and trade-offs of doing multiple PCI transactions.