app/testpmd: refine xstats show

app/testpmd: refine xstats show

When using "show port xstats all" command to show xstats, the output
is usually too long to obtain what you really want, especially when
multi-queue is enabled.

Added an option to set whether zero values should be displayed
or not for xstats. The "set xstats-hide-zero on|off" command enables
the user to decide if the zero values should be shown while
displaying xstats.

Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Signed-off-by: Elza Mathew <elza.mathew@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>