Add virtkey lib for usage-improvment and keycode translating.
Add 4 internal API for the aim

const char *virKeycodeSetTypeToString(int codeset);
int virKeycodeSetTypeFromString(const char *name);
int virKeycodeValueFromString(virKeycodeSet codeset, const char *keyname);
int virKeycodeValueTranslate(virKeycodeSet from_codeset,
                             virKeycodeSet to_offset,
                             int key_value);

* include/libvirt/libvirt.h.in: extend virKeycodeSet enum
* src/Makefile.am: add new virtkeycode module and rule to generate
  virkeymaps.h
* src/util/virkeycode.c src/util/virkeycode.h: new module
* src/util/virkeycode-mapgen.py: python generator for virkeymaps.h
  out of keymaps.csv
* src/libvirt_private.syms: extend private symbols for new module
* .gitignore: add generated virkeymaps.h