mem_access: mem event additions for access

mem_access: mem event additions for access

* Adds an ACCESS memory event type, with RESUME as the action.

* Refactors the bits in the memory event to store whether the memory event
  was a read, write, or execute (for access memory events only).  I used
  bits sparingly to keep the structure somewhat the same size.

* Modified VMX to report the needed information in its nested page fault.
  SVM is not implemented in this patch series.

Signed-off-by: Joe Epstein <jepstein98@gmail.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Tim Deegan <Tim.Deegan@citrix.com>