v2: Make sure we only abort PIO commands if BCL is zero, not DMA.

________________________________________________________________________________

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

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