mem_access: added INT3/CRx capture

mem_access: added INT3/CRx capture

* Allows a memory event listener to register for events on changes to
  CR0, CR3, and CR4, as well as INT3 instructions, as a part of the
  mem_access mechanism.  These events can be either synchronous or
  asynchronous.

* For INT3, the logic works independent of a debugger, and so both can
  be supported.

* The presence and type of listener are stored and accessed through
  HVM params.

* Changed the event mask handling to ensure that the right events are
  captured based on the listeners.

* Added the ability to inject HW/SW traps into a VCPU when it next
  resumes (rather than try to modify the existing IRQ injection
  code paths).  Only one trap to inject can be outstanding at a time.

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