MFC 261000,261785,263238,263322,264302:
Various AHCI fixes:

• Fix issue with stale fields from a recycled request pulled off the freelist.
• Provide an indication a "PIO Setup Device to Host FIS" occurred while executing the IDENTIFY DEVICE and IDENTIFY PACKET DEVICE commands.
• Provide an indication a "D2H Register FIS" occurred during a SET FEATURES command.
• Though there currently isn't a way to insert new media into an ATAPI drive, at least pretend to support Asynchronous Notification (AN) to avoid a guest needlessly polling for it.
• Don't reissue in-flight commands.
• Constrain the amount of data returned to what is actually available not the size of the buffer.