Implement legacy interrupts for the AHCI device emulation
according to the method outlined in the AHCI spec.

Tested with FreeBSD 9/10/11 with MSI disabled,
and also NetBSD/amd64 (lightly).

Reviewed by:	neel, tychon
MFC after:	3 weeks
Set the new floating point exception mask correctly

Submitted by:	Keith White <kwhite@site.uottawa.ca>