Events: add MSR write monitoring for Xen

This PR provides support for the new Xen 4.3 feature enabling observation of MSR writes.

Also included: 
1) an example program demonstrating this capability
2) improved sanity checking for event initialization in VMI_INIT_COMPLETE mode
3) events initialization during VMI_INIT_PARTIAL (necessary for observing very early MSR events)
3) finally, some notes showing MSR writes during boot for Centos6 x86-64 and WinXP.