need interrupt routing fallback/heuristic for broken MPtable
It seems that many systems come with a broken MPtable. For instance my machine
at work misses an entry for bus 4 (where my NIC is on), but instead contains an
entry for the non-existing bus 253.
One possible heuristic would be to assume that if there is one bus not mentioned
in the MPtable, but also one bus of the MPtable not existing in the system, that
this bus entry in the MPtable actually corresponds to the missing entry. I am
not sure how good this might work though.
Alternatively, pci-pci bridges to buses which are not mentioned in the MPtable
could be treated as pci-pci bridges on physical slots, i.e. we could assume