trivial: Fix compiler warnings

- Add missing return statements
- Add missing const modifiers
- Initialize variables correctly
- Remove unused variables
- Correct incorrect type definitions
- Use correct printf format string types

x86-iospace-dma: Remove camkes_iommu_dma_alloc

This function is dead code and can be removed.

Add x86-iospace-dma module

This module maps all of DMA into a provided IOSpace. This enables
devices to be restricted via an iommu transparently from the drivers
perspective.