conf: start search for next unused PCI address at same slot as previous find
    
    There is a very slight time advantage to beginning the search for the
    next unused PCI address at the slot *after* the previous find (which
    is now used), but if we do that, we will miss allocating the other
    functions of the same slot (when we implement a
    VIR_PCI_CONNECT_AGGREGATE_SLOT flag to support that).