MFH: r259647

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.