Acquire node allocation lock during node query

If locking is used (usually by ganeti-watcher), node allocations must be
temporarily blocked.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>

lib/cmdlib.py

@@ -5227,6 +5227,7 @@ class _NodeQuery(_QueryBase):
     if self.do_locking:
       # If any non-static field is requested we need to lock the nodes
       lu.needed_locks[locking.LEVEL_NODE] = self.wanted
+      lu.needed_locks[locking.LEVEL_NODE_ALLOC] = locking.ALL_SET
   def DeclareLocks(self, lu, level):
     pass