Resurrect the events branch with updates for Xen 4.2.0 and miscellaneous fixes. 

This code is 95% the work of Nassir Salim. I've updated it, refactored some components, added some missing features, and expanded the events example.

Changes from the events branch originally hosted at Google code
        1) Refactor to support Xen 4.2 (and Xen 4.1 mem_events). This may be simplified to be Xen 4.2 only, per discussion with Bryan and Nassir. However, there are new mem_events features headed into 4.3 that warrant retention of the checks in the configure.ac
        2) Add vcpu ID to mem_event structure
        3) Add int3 handler
        4) A handful of bugfixes relative to the HVM_onchangeonly flag (e.g. it needed to be OR'd into the flag, rather than assignment)
        5) General refactoring to track the upstream xen-access.c mem_events example
        6) A bit of a rewrite of the event_example.c code
        7) Fix unconditional initialization of memory events. (Now only upon request and IFF hvm).

    Changes that are necessary in the future include improved event registration functionality per discussion on vmitools mailing list and via email

    Important enhancements that should be pursued:
        1) detection of EPT to validate that events are possible to initialize.
        2) determination of AMD support using RVI
        3) support for the newly added MSR write monitoring (to appear in Xen 4.3)
        4) possibility of a KVM equivalent of memory events