x86: remove IS_PRIV_FOR references

x86: remove IS_PRIV_FOR references

The check in guest_physmap_mark_populate_on_demand is redundant, since
its only caller is populate_physmap whose only caller checks the
xsm_memory_adjust_reservation hook prior to calling.

Add a new XSM hook for the other two checks since they allow privileged
domains to arbitrarily map a guest's memory.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com> (release perspective)