mem_access: Add helper API to setup ring and enable mem_access

mem_access: Add helper API to setup ring and enable mem_access

tools/libxc: Add helper function to setup ring for mem events
This patch adds a helper function that maps the ring, enables mem_event
and removes the ring from the guest physmap while the domain is paused.
This can be used by all mem_events but is only enabled for mem_access at
the moment.

tests/xen-access: Use helper API to setup ring and enable mem_access
Prior to this patch, xen-access was setting up the ring page in a way
that would give a malicous guest a window to write in to the shared ring
page. This patch fixes this by using the helper API that does it safely
on behalf of xen-access.

This is XSA-99.

Signed-off-by: Aravindh Puthiyaparambil <aravindp@cisco.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>