mem_event: Allow memory access listener to perform single step execution.

mem_event: Allow memory access listener to perform single step execution.

Add a new memory event that handles single step. This allows the
memory access listener to handle instructions that modify data within
the execution page.  This can be enabled in the listener by doing:
xc_set_hvm_param(xch, domain_id, HVM_PARAM_MEMORY_EVENT_SINGLE_STEP,
HVMPME_mode_sync)

Now the listener can start single stepping by:
xc_domain_debug_control(xch, domain_id,
XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_ON, vcpu_id)

And stop single stepping by: xc_domain_debug_control(xch, domain_id,
XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_OFF, vcpu_id)

Signed-off-by: Aravindh Puthiyaparambil <aravindh@virtuata.com>
Acked-by: Tim Deegan <Tim.Deegan@citrix.com>