virt: Backup vm xml to a desired directory, and recover vm
from a desired xml

1.Add a variable(self.xml_file) to VM class
2.Modify backup_xml function, so it can backup to a desired
  directory or self.xml_file.
3.Add recover_from_xml function, so it can recover vm from a
  desired xml file or self.xml_file.

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