ruby: Implement SwapReq support for Ruby memory
This implements SwapReq for Ruby memory.
A SwapReq should be treated like a write, except that the response

packet contains the overwritten data.
Note that, in particular, the conditional checking for isStore/isLoad

needs to be reversed, as a SwapReq is both.

Quick regressions.
The workload requiring this feature now works: http://reviews.gem5.org/r/3449/