xsm: add hooks for claim

xsm: add hooks for claim

Adds XSM hooks for the recently introduced XENMEM_claim_pages and
XENMEM_get_outstanding_pages operations, and adds FLASK access vectors
for them. This makes the access control decisions for these operations
match those in the rest of the hypervisor.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com> (for 4.3 release)
Acked-by: Keir Fraser <keir@xen.org>