Initial interrupt setup code.

handler doesn't do anything yet.
start translation at 16mb.

two reasons really for this:
1) sg_dma assumes that an address of 0 is an error.
2) isa ranges.

we can reevaluate later if appropriate.
Set up dma tag if the drhd is enabled.
code to make interrupts fire.

yet to manage to get this to actually trigger in my test setup, but
should be mostly there...