If we changes the IDE lun for CD/DVD and OS disk, sometimes the boot
will fail because of failing to attach the disk device. There are two reasons:
- stordisengage driver does not work as we expected (disable the loading of ATA driver).
It failed to stop the loading of ATA drivers.
- if the storvsc find an invalid IDE lun, reporting CAM_SEL_TIMEOUT to
CAM layer is correct.
This fix disabled ATA driver if FreeBSD is started on Hyper-V VM.
Submitted by: Hongjiang Zhang <honzhan microsoft com>