dpif: fix dpctl "show" command

Modify dpif-dpdk such that the dpctl show command reports
the correct number of flows. As well as this, for the statistics
we don't maintain, we represent these with question marks, rather
than invalid figures.

Change-Id: Ic3195323cf092da84a5970fc42848e50c82ec9ae
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Reviewed-by: Kevin Traynor <kevin.traynor@intel.com>
Reviewed-by: Stephen Finucane <stephen.finucane@intel.com>