Have lookup_nvlist() function return an NvListRef instead of an NvList

[don-brady]

This is a request to have the lookup_nvlist() function return an NvListRef instead of an NvList. The returned nvlist is not a copy nor do I own it or should be allowed to pass ownership to a function which could drop it. An NvListRef seems a more appropriate type to return.

Same for lookup_nvlist_array() which should probably return a Vec<NvListRef>.

Note that for an nvpair, the data() function returns an NvListRef when the pair is an nvlist.