The ahci code was missing its soft reset functionality. This wasn't really an
issue for Linux guests, but Windows gets confused when the controller doesn't
reset when it tells it so.

Using this patch I can now successfully boot Windows 7 from AHCI using AHCI
enabled SeaBIOS.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>