[Libguestfs] [PATCH libnbd] info: Add --map --totals sub-mode to display summary

This adds an nbdinfo sub-option --map --totals to display the total of each type of mapping: $ nbdkit -r file fedora-33.img --run 'nbdinfo --map --totals $uri' 1226113024 19.0 0 data 5216337920 81.0 3 hole,zero I need this kind of functionality for virt-v2v in order to estimate how much space is required on the target, although for virt-v2v I will probably use the API directly. Nevertheless it seems like a useful feature and the implementation does not complicate nbdinfo. It can also be used for other map types, eg. for showing how much of the dirty-bitmap is dirty. The implementation could be done in a single pass although for the vast majority of use cases I doubt that would be any faster since the table easily fits in L1 cache, and it would require a complex data structure. But it's worth noting that a malicious server could try to make nbdinfo use O(n^2) time (with a very small constant). Rich.