mem_access: modifications to mem_event enable API.

tools/libxc/:
1. Modified the API xc_mem_event_enable to initialize
   shared ring to communicate with the hypervisor along with enabling mem_event.
2. Added memset to clear the ring_page of any bogus input before enabling any events.
3. Replaced calls to deprecated function xc_map_foreign_batch with calls
   to xc_map_foreign_bulk. The function xc_map_foreign_bulk has a cleaner
   error reporting interface than xc_map_foreign_batch.
4. The API xc_mem_event_enable is now modified to return int rather
   than void *,this was done to synchronize this API's behaviour with other
   mem_event API's.

tools/tests/xen-access/: Updated code to use the new helper API.

tools/ocaml/libs/xc/xenctrl_stubs.c: Changed the name of a macro from RING_SIZE
to BUF_RING_SIZE because the earlier name collided with xen shared ring
deinitions in io/ring.h

Signed-off-by: Dushyant Behl <myselfdushyantbehl@gmail.com>