mem_event: Added new helper API to teardown mem event setup and unmap ring_page.

tools/libxc/xc_mem_event.c: Added new generic API to teardown mem event setup,
the API supports hvm params PAGING, ACCESS and SHARING and also completes the
obvious job of unmapping ring_page.

tools/libxc/xc_mem_access.c: Modified mem_event_disable to use the new teardown
API.

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

Signed-off-by: Dushyant Behl <myselfdushyantbehl@gmail.com>
xenpaging: updated code to use safer mem_event API's for setup and teardown. 

tools/libxc/xc_mem_paging.c: updated mem_paging enable and disable API's to use
the mem event enable and disable routines. The mem event API's take care of
security issues mentioned in XSA-99 and also provide more coarse grained
behaviour.

tools/xenpaging/xenpaging.c: added calls to the new API's and removed the code
which duplicated the new API behaviour.

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