setting reserveEntireSlot really accomplishes nothing - instead of
going to the trouble of computing the value for reserveEntireSlot and
then possibly setting *all* functions of the slot as in-use, we can
just set the in-use bit only for the specific function being used by a
device.  Later we will know from the context (the PCI connect flags,
and whether we are reserving a specific address or asking for "the
next available") whether or not it is okay to allocate other functions
on the same slot.

Although it's not used yet, we allow specifying "-1" for the function
number when looking for the "next available slot" - this is going to
end up meaning "return the lowest available function in the slot, but
since we currently only provide a function from an otherwise unused
slot, "-1" ends up meaning "0".