Move the parsing of each <primitive> to a method
Simplify the parsing of <primitive> elements

The previous implementation had a lot of duplication of expressions, and
pointless creation of nested hashes with only one element only to later unpack
them with slightly different keys.
Eliminate repeated code to parse <nvpair>s
Merge pull request #41 from bitglue/3394_utilization

make primitive utilization attributes managable