libxml2 : convert to new staging and fix python error in gnome-doc-utils

libxml2 : convert to new staging and fix python error in gnome-doc-utils

Converting libxml2 recipes to the new staging way
  fixed the error where,during the compilation of gnome-doc-utils
  libxml2.py couldn't be find,but strangely the libxml package had
  only libxml.py

Also removed the following useless part as suggested by Phil Blundell
  on mailing list:
       install -d ${STAGING_DATADIR}/aclocal/
       install -d ${STAGING_BINDIR_CROSS}