xenvm: drivers: xen: add Xen grant table driver

This commit introduces driver for granting access for own grant
table and for mapping/unmapping foreign gref. Grant tables are used
for data exchange between Xen domains via shared memory page(s) (e.g.
for sharing ring buffer with driver data) This functionality is
widely used and needed for implementing PV backend/frontend drivers.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>