add ahci support

add ahci support

This patch adds AHCI support to seabios.  Tested with virtual hardware
only (upcoming ahci support in qemu).  Coded by looking at the
recommandations in the intel ahci specs, so I don't expect much trouble
on real hardware.  Tested booting fedora install from hard disk and a
opensuse live iso from cdrom.

[ v2: disable by default           ]
[ v2: add check for malloc failure ]
[ v2: wind up disk write support   ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>