conf: pay attention to bus minSlot/maxSlot when autoassigning PCI addresses

conf: pay attention to bus minSlot/maxSlot when autoassigning PCI addresses

The function that auto-assigns PCI addresses was written with the
hardcoded assumptions that any PCI bus would have slots available
starting at 1 and ending at 31. This isn't true for many types of
controllers (some have a single slot/port at 0, some have slots/ports
from 0 to 31). This patch updates that function to remove the
hardcoded assumptions. It will properly find/assign addresses for
devices that can only connect to pcie-(root|downstream)-port (which
have minSlot/maxSlot of 0/0) or a pcie-switch-upstream-port (0/31).

It still will not auto-create a new bus of the proper kind for these
connections when one doesn't exist, that task is for another day.