xen: Introduce shared buffer helpers for page directory...

based frontends. Currently the frontends which implement
similar code for sharing big buffers between frontend and
backend are para-virtualized DRM and sound drivers.
Both define the same way to share grant references of a
data buffer with the corresponding backend with little
differences.

Move shared code into a helper module, so there is a single
implementation of the same functionality for all.

This patch introduces code which is used by sound and display
frontend drivers without functional changes with the intention
to remove shared code from the corresponding drivers.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>