utils: Optimize namedtuple definition in pidStat

Profiling idle vdsm reveal that 60% of cpu time is spent in creating
pointless namedtuples. In particular, 823 calls were made in 4.88
seconds of cpu time, invoking collections.py genexpr function 296805
times.

This patch creates the one and only namedtuple needed by pidStat when
the module is imported.

Change-Id: Ic5841b338cc0e9376c3bd7815206c27829c286d6
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26097
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>