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Add tests to validate processing of descriptors and read/write data.

The design is based on the virtio_queue::mock::MockSplitQueue
implementation, which is used to add a set of descriptors to the memory.
The same memory is then processed via DescriptorChain and the same set
of descriptors magically appear.

This patch adds various tests for success and failure and also validate
the data read or written.
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>