Prepare the memory for an I/O transfer.
The direction of the I/O just completed, or kIODirectionNone for the direction specified by the memory descriptor.
An IOReturn code.
This involves paging in the memory, if necessary, and wiring it down for the duration of the transfer. The complete() method completes the processing of the memory after the I/O transfer finishes. Note that the prepare call is not thread safe and it is expected that the client will more easily be able to guarantee single threading a particular memory descriptor.