%ifutil: Add NIC utilization percentage to sar network stats

This patch adds %ifutil to statistics displayed by sar -n.
%ifutil is a new metric giving the utilization percentage of the
corresponding network interface.
%ifutil: Update sadf output to take into account the new metric

This patch updates the various output formats of sadf (CSV, ppc, JSON,
XML) to take into account the new %ifutil metric added to sar network
devices statistics.
DTD and XSD documents have also been updated accordingly.