This introduces a FeatureWord enum, FeatureWordInfo struct (with
generation information about a feature word), and a FeatureWordArray
typedef, and changes add_flagname_to_bitmaps() code and
cpu_x86_parse_featurestr() to use the new typedefs instead of separate
variables for each feature word.
This will help us keep the code at kvm_check_features_against_host(),
cpu_x86_parse_featurestr() and add_flagname_to_bitmaps() sane while
adding new feature name arrays.