add an experimental implementation of PCI MSIs (Message Signaled
Interrupts). Successfully tested with hdaudio and "wpi" wireless
ethernet.
notes:
-There seem to be buggy chips around which announce MSI support
but don't correctly implement it. Thus the final word whether MSIs
can be used should be by the driver.
-Only a single vector is supported. For multiple vectors, the IDT
allocation code would have to be changed. (And we would possibly
run into problems due to the limited number of vectors supported
by the current code.)
-The code is "#if NIOAPIC > 0" because it uses the ioapic_edge
interrupt stubs. These actually don't touch any ioapic, so this
is somewhat a misnomer.
-MSIs can't be identified by a "pin" but only by a cpu/vector
pair. Common intr code soesn't deal well with this yet.
-Drivers need to take care of saving/restoring MSI data in the device's
config space on suspend/resume.

(drochner)