trivial: Style changed file

trivial: Fix compiler warnings

- Remove unused variables
- Conditionally define functions that are conditionally used
- Add default cases to non-exhaustive switch statements
- Correct printf modifiers
- Correct type mismatches
- Initialize variables to prevent uninitialized usages
- Insert missing return statements

libsel4camkes: Set virtqueue size on creation

The queue size of a virtqueue is how many entries its rings and
descriptor table have.

msgqueue: Remove unnecessary volatile qualifiers

The qualifiers aren't necessary to restrict the behaviour of the
compiler and can be removed.

libsel4camkes: Add msgqueue

The msgqueue allows the passing of messages from one component
to another. It is essentially a layer on top of the CAmkES virtqueues,
in which it sends small buffers that is filled with information about an
message to a receiver.