Correct the offset to adjust for IPMIv2
Correct the offset to adjust for IPMIv2

For IPMIv2 sessionless data, there is not 13 bytes
to strip out, but instead only 2.  I suspect because
reading the sample packet, the last row unique values
were added without looking at the context.  Correcting
for context, the IPMIv2 header is only two bytes longer.

Change-Id: I9b99417c39f519dfa68b59a90a3ef7ca0232e97e

openstack/pyghmi,openstack/pyghmi,stackforge/pyghmi