Summary

1. Change the MCA board id from 0x4d to 0x4f again (though need to see an option rom for this, damn).
2. Reworked the way IRQ's are issued in the 53c9x MCA side.
3. Implemented Selection and ATN 3 command (0x46), which uses 3 message bytes rather than just 1. Fixes boot issues with the PCSCSI drivers for NT 3.1.
4. Ported the latest qemu commits based on this chip.

Checklist

• Closes #xxx
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/
• This pull request requires changes to the asset set
  • I have opened an assets pull request - https://github.com/86Box/assets/pull/changeme/

References

AMD PCscsi manual (non-A counts too)