Use xpath expression to find 'directory' element.

Fix exception when path contains routing engine 're1:/var/tmp'.
>>> from jnpr.junos.utils.fs import FS
>>> fs = FS(dev)
>>> x = fs.ls('re1:/var/tmp')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serg/src/venv/local/lib/python2.7/site-packages/jnpr/junos/utils/fs.py", line 213, in ls
    if not xdir.get('name'):
AttributeError: 'NoneType' object has no attribute 'get'