iscsi: do not fail to stop a stopped pool

Just as we allow stopping filesystem pools when they were unmounted
externally, do not fail to stop an iscsi pool when someone else
closed the session externally.

Reported at:
https://bugzilla.redhat.com/show_bug.cgi?id=1171984

src/storage/storage_backend_iscsi.c

@@ -449,8 +449,13 @@ virStorageBackendISCSIStopPool(virConnectPtr conn ATTRIBUTE_UNUSED,
                                virStoragePoolObjPtr pool)
 {
     char *portal;
+    char *session;
     int ret = -1;
 
+    if ((session = virStorageBackendISCSISession(pool, true)) == NULL)
+        return 0;
+    VIR_FREE(session);
+
     if ((portal = virStorageBackendISCSIPortal(&pool->def->source)) == NULL)
         return -1;