conf: eliminate virDomainPCIAddressReserveNextSlot()
    
    Since we don't actually reserve an entire slot at a time anymore, the
    name of this function is just confusing, and it's almost identical in
    operation to virDomainPCIAddressReserveNextAddr() anyway, so remove
    the *Slot() function and replace calls to it with calls to *Addr(...,
    -1).