maint: Rename xmlconf to xmlopt and virDomainXMLConfig to virDomainXMLOption

maint: Rename xmlconf to xmlopt and virDomainXMLConfig to virDomainXMLOption

This patch is the result of running:

for i in $(git ls-files | grep -v html | grep -v \.po$ ); do
  sed -i -e "s/virDomainXMLConf/virDomainXMLOption/g" -e "s/xmlconf/xmlopt/g" $i
done

and a few manual tweaks.