ATAPI transfers using the AHCI HBA will work if you ask the HBA to perform
a DMA transfer, but if you ask for PIO, it will not.

All AHCI transfers are actually DMA anyway, but the AHCI HBA will need some
assistance from the core layer to help it manage its covert-ops DMA when
performing PIO routines deep in the bowels of the ATAPI code.

This is strictly a bugfix and /maybe/ it's 2.4-rc1 material, but the CDROM
tests for AHCI are not ready for prime-time (we don't have ANY cdrom tests
yet, really!) so I am submitting just the fix now, and my say-so that yes,
this fixes OVMF booting and pleases my hacky, off-list CDROM qtests.
________________________________________________________________________________

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

This version is tagged ahci-atapi-pio-v1:
https://github.com/jnsnow/qemu/releases/tag/ahci-atapi-pio-v1