xen_backend: add an emulation of grant copy

xen_backend: add an emulation of grant copy

Not all Xen environments support the xengnttab_grant_copy() operation.
E.g. where the OS is FreeBSD or Xen is older than 4.8.0.

This patch introduces an emulation of that operation using
xengnttab_map_domain_grant_refs() and memcpy() for those environments.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>