AHCI is supported on various VMM, e.g. Virtual Box, VMware Workstation.
Adding AHCI support enables OSv to run on them if the para-virtualized
block device is not present or not supported yet.

Tested on VirtualBox, VMware Workstation and QEMU.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>