AHCI device model in userspace for bhyve

by Zhixiang Yu for FreeBSD

Currently bhyve only supports virtio disk for the guest's block device. This project will add AHCI device emulation to bhyve so that we can emulate normal cdroms and disks. This project will benefit bhyve a lot. First of all, since AHCI is widely supported in various Operating Systems, bhyve can support other nonproprietary and proprietary guest OSs without the virtio disk driver in those OSs. Secondly, this project will make it possible to install a GENERIC system from a emulated cdrom device.