@yumingfei, Sorry I didn't make it clear.

It works fine if the XML contains <numatune> entry.

But when dump an XML from an VM whose placement is auto and mode is strict, 
the numa entry is not shown in this XML. 

So when libvirt_xml parsing a config dumped from such a VM, it raises an exception:

```
11:19:41 ERROR| Traceback (most recent call last):
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/standalone_test.py", line 195, in run_once
11:19:41 ERROR|     run_func(self, params, env)
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/libvirt/tests/src/virsh_cmd/domain/virsh_numatune.py", line 211, in run_virsh_numatune
11:19:41 ERROR|     set_numa_parameter(params)
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/libvirt/tests/src/virsh_cmd/domain/virsh_numatune.py", line 141, in set_numa_parameter
11:19:41 ERROR|     if check_numatune_xml(params):
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/libvirt/tests/src/virsh_cmd/domain/virsh_numatune.py", line 42, in check_numatune_xml
11:19:41 ERROR|     numa_params = libvirt_xml.VMXML.get_numa_params(vm_name)
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/libvirt_xml/vm_xml.py", line 491, in get_numa_params
11:19:41 ERROR|     return vmxml.numa
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/propcan.py", line 223, in __getattr__
11:19:41 ERROR|     return self.__getitem__(key)
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/propcan.py", line 183, in __getitem__
11:19:41 ERROR|     return accessor()
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/libvirt_xml/accessors.py", line 618, in __call__
11:19:41 ERROR|     self.tag_name, create=False)
11:19:41 ERROR|   File "/home/celeron/Workspace/virt-test/virttest/libvirt_xml/accessors.py", line 161, in element_by_parent
11:19:41 ERROR|     raise xcepts.LibvirtXMLAccessorError(excpt_str)
11:19:41 ERROR| LibvirtXMLAccessorError: Exception thrown from Getter for property "numa" while looking for element tag "memory", on parent at xpath "numatune", in XML
```