XMLDesc requires a flags parameter

XMLDesc uses the VIR_DOMAIN_XML_* flags, but in this case we don't want to use any of the flags, so we pass in 0.

Without using the flags parameter, the following traceback is thrown:
```
Traceback (most recent call last):
  File "/tmp/virt-hosts", line 82, in <module>
    main()
  File "/tmp/virt-hosts", line 53, in main
    desc = etree.fromstring(dom.XMLDesc())
TypeError: XMLDesc() takes exactly 2 arguments (1 given)
```