AHCI emulation

by Octachoron for QEMU development team

Advanced Host controller Interface (AHCI) replaces early Serial ATA controller interfaces that either used proprietary extensions to enable native features, or omitted support completely. Being unified, most modern operating systems include AHCI drivers. It is also a faster alternative to IDE emulation. Therefore, the addition of AHCI emulation to QEMU, along with Serial ATA command interpretation, would improve the usefulness of QEMU significantly.