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.