(ghost/spec) pkvm_host_map_guest only copying the pkvm state when needed

when no memcache donation happened during the hcall, the pkvm lock
will not have been taken (and therefore there is nothing to copy).

Author: kmemarian

arch/arm64/kvm/hyp/nvhe/ghost_spec.c

@@ -670,7 +670,12 @@ bool compute_new_abstract_state_handle___pkvm_host_map_guest(struct ghost_state
 	 * so it's safe to copy.
 	 */
 	copy_abstraction_host(g1, g0);
-	copy_abstraction_pkvm(g1, g0);
+	if (call->memcache_donations.len > 0)
+		copy_abstraction_pkvm(g1, g0);
+	else {
+		ghost_assert(!g0->pkvm.present);
+		g1->pkvm.present = false;
+	}
 
 	for (int d=0; d<call->memcache_donations.len; d++) {
 		u64 pfn = call->memcache_donations.pages[d];