MFC r340092 (by imp): Implement ability to turn on/off PHYs for AHCI devices.

As part of Chuck's work on fixing kernel crashes caused by disk I/O
errors, it is useful to be able to trigger various kinds of
errors. This patch allows causing an AHCI-attached disk to disappear,
by having the driver keep the PHY disabled when the driver would
otherwise enable the PHY. It also allows making the disk reappear by
having the driver go back to setting the PHY enable/disable state as
it normal would and simulating the hardware event that causes a bus
rescan.