virsh_nodeinfo: Alter algorithm for cpu frequency

Obtaining the /proc/cpuinfo at two different points in time
can result in slightly different values for 'CPU frequency'.
So rather than just straight out fail because they don't match
exactly, allow for a 20% variation of either value before
declaring failure
Merge pull request #1007 from jferlan/virsh_nodeinfo

virsh_nodeinfo: Alter algorithm for cpu frequency