Use page directory based shared buffer implementation
now available as common code for Xen frontend drivers.

Remove flushing of shared buffer on page flip as this
workaround needs a proper fix.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>