conf: permit auto-assignment of controller indexes

conf: permit auto-assignment of controller indexes

Hand-entering indexes for 20 PCI controllers is not as tedious as
manually determining and entering their PCI addresses, but it's still
annoying, and the algorithm for determining the proper index is
incredibly simple (in all cases except one) - just pick the lowest
unused index.

The one exception is USB2 controllers because multiple controllers in
the same group have the same index. For these we look to see if 1) the
most recently added USB controller is also a USB2 controller, and 2)
the group *that* controller belongs to doesn't yet have a controller
of the exact model we're just now adding - if both are true, the new
controller gets the same index, but in all other cases we just assign
the lowest unused index.

With this patch in place and combined with the automatic PCI address
assignment, we can define a PCIe switch with several ports like this:

  <controller type='pci' model='pcie-root-port'/>
  <controller type='pci' model='pcie-switch-upstream-port'/>
  <controller type='pci' model='pcie-switch-downstream-port'/>
  <controller type='pci' model='pcie-switch-downstream-port'/>
  <controller type='pci' model='pcie-switch-downstream-port'/>
  <controller type='pci' model='pcie-switch-downstream-port'/>
  <controller type='pci' model='pcie-switch-downstream-port'/>
  ...

These will each get a unique index, and PCI addresses that connect
them together appropriately with no pesky numbers required.