Design a mocking framework for virtio queues

[andreeaflorescu]

As part of this task we want to define what & how we want to implement the mocking framework. The delivery of this task is a design document that can be used to further break down #61.

[krady21]

Instead of writing a mock framework from scratch, we decided to add two helper functions:

• The first one would allow us to more easily create descriptor chains by adding a new structure called `MockDescriptor. This struct would provide users with a way to incrementally create descriptors, similar to how a builder pattern works.
• The second one would allow us to write a buffer (&[u8]) to the underlying memory regions that a chain of descriptors point to.