tools/power turbostat: Return default value for unmapped PMT domains

When requesting PMT counters with --add command, user may want to skip
specifying values for all the domains (that is, cpu, core, package etc).
For the domains that user did not provide information on how to read the
counter, return default value - zero.

Signed-off-by: Patryk Wlazlyn <patryk.wlazlyn@linux.intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>