Add ATAPI support into libqos/ahci, and write a few tests for it.

This is the last "batch" of planned qtests for s/ata -- basic i/o
testing of HDDs and CDROMs on both PCI and AHCI should be complete
after this series.

v2: Rebase and resend for 2.6.

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

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