conf: eliminate virDomainPCIAddressReserveNextSlot()

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).