This is a small handful of fixes for the ahci-tests/ahci device,
alongside two new tests. I have a larger series of NCQ patches
coming shortly, but these patches were unrelated so I cleaned them
up and am sending them out separately.

v2:
 - Fixed unaligned 2/4 byte read logic
 - Tidied comments and justification for patch 1.

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For convenience, this branch is available at:
https://github.com/jnsnow/qemu.git branch ahci-tests-2.4
https://github.com/jnsnow/qemu/tree/ahci-tests-2.4

This version is tagged ahci-tests-2.4-v2:
https://github.com/jnsnow/qemu/releases/tag/ahci-tests-2.4-v2