Import Zhixiang Yu's GSoC'13 AHCI emulation:

https://wiki.freebsd.org/SummerOfCode2013/bhyveAHCI

This provides ICH8 SATA disk and ATAPI ports, selectable
via the bhyve slot command-line parameter:

SATA

-s <slot>,ahci-hd,<image-file>

ATAPI

-s <slot>,ahci-cd,<image-file>

Slight modifications by: grehan@
Approved by: re@ (blanket)
Obtained from: FreeBSD GSoC'13