Ensure cpu[:real] is not 0. Add real_cores attribute. OHAI-292

Using an AWS instance with one CPU as a test case, /proc/cpuinfo does not return a 'physical id' or 'cpu cores'. As a result, cpu[:real] is set to 0. Line 60 ensures that cpu[:real] will default to 1 in this case.

cpu[:real_cores] is a new attribute that satisfies http://tickets.opscode.com/browse/OHAI-292. This provides a total of the real cores available to a system, ignoring the inflated count of cpu[:total] that results from hyperthreading.