Make force_migrate not replace existing node

Previous to this, if once force-migrated a VM, the previous_node value
would be updated to the current node, which is likely never what an
administrator would want. Change this functionality so that the previous
node value is not changed, and update the documentation to reflect this.

client-common/vm.py

@@ -444,6 +444,10 @@ def migrate_vm(zk_conn, domain, target_node, selector, force_migrate, is_cli=Fal
             common.stopZKConnection(zk_conn)
             return False, 'ERROR: VM "{}" is already running on node "{}".'.format(domain, current_node)
 
+    # Don't overwrite an existing last_node when using force_migrate
+    if last_node and force_migrate:
+        current_node = last_node
+
     zkhandler.writedata(zk_conn, {
         '/domains/{}/state'.format(dom_uuid): 'migrate',
         '/domains/{}/node'.format(dom_uuid): target_node,