xen/granttable: Support transitive grants

These allow a domain A which has been granted access on a page of domain B's
memory to issue domain C with a copy-grant on the same page.  This is useful
e.g. for forwarding packets between domains.

Signed-off-by: Annie Li <annie.li@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>