xs on Series with MultiIndex gives KeyError
```
In [3]: df
Out[3]:
                    A  B  C  D
first second third
bar   one    1      4  1  8  9
      two    1      7  5  5  0
baz   one    1      6  6  8  0
      three  2      5  3  5  3

[4 rows x 4 columns]

In [4]: df.xs('one', level=1)
Out[4]:
             A  B  C  D
first third
bar   1      4  1  8  9
baz   1      6  6  8  0

[2 rows x 4 columns]

In [5]: df[['A']].xs('one', level=1)
Out[5]:
             A
first third
bar   1      4
baz   1      6

[2 rows x 1 columns]

In [6]: df['A'].xs('one', level=1)
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-11-62c0cf8731b1> in <module>()
----> 1 df['A'].xs('one', level=1)

C:\Anaconda\lib\site-packages\pandas\core\series.pyc in _xs(self, key, axis, lev
el, copy)
    437
    438     def _xs(self, key, axis=0, level=None, copy=True):
--> 439         return self.__getitem__(key)
    440
    441     xs = _xs

C:\Anaconda\lib\site-packages\pandas\core\series.pyc in __getitem__(self, key)
    482     def __getitem__(self, key):
    483         try:
--> 484             result = self.index.get_value(self, key)
    485             if isinstance(result, np.ndarray):
    486                 return self._constructor(result,index=[key]*len(result))
.__finalize__(self)

C:\Anaconda\lib\site-packages\pandas\core\index.pyc in get_value(self, series, k
ey)
   2294                     raise InvalidIndexError(key)
   2295                 else:
-> 2296                     raise e1
   2297             except Exception:  # pragma: no cover
   2298                 raise e1

KeyError: 'one'
```
