o Remove assertions on ipa_version as sometimes the version detection
using cpuid can be quirky (this is the case of VMWare without the vPMC support) but fail to probe hwpmc.
o Apply the fix for XEON family of processors as established by
315338-020 document (bug AJ85).
Sponsored by: EMC / Isilon storage division
Reviewed by: fabient
MFC courtesy of panzura.