virt: Backup vm xml to a desired directory.

1.If no directory passed to function, xmlfile will be created in
  data_dir.get_tmp_dir() and removed automatically when test is over.
2.If a desired directory passed to function, xmlfile will be created
  in this directory, won't be removed if directory always exists.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>