rtio: Callback chaining and testing

Callbacks were a bit neglected in terms of test coverage, especially
when used in chains. It was clear from the code that chained callbacks
may not actually work, and callback ordering then was hard to verify.
Test callbacks chained to transactions work as expected.

The test iodev had built up some cruft over time and in the process
showed a few bugs once callback chaining was fixed so the test iodev now
better matches typical iodev implementations at this point.

Cancellation testing now includes an added case for cancelling a the
second submission in the chain prior to calling submit noting that no
completions notifications should be given back for those.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>