Allow use of unicode characters when searching for VMs by name

Hi, 

I came across your library and it seems like a very good fit for testing some features of the product I am working on. We have a requirement to support VMs with unicode characters in their names, and this patch addresses the following warning (from pysphere 0.1.8) when such VMs exist:

```
./usr/local/lib/python2.7/dist-packages/pysphere/vi_server.py:317: UnicodeWarning: Unicode equal comparison failed to convert both arguments to Unicode - interpreting them as being unequal
  if v == name:
```

The problem is that pysphere deals with utf8-encoded strings rather than unicode strings, so this patch utf8-encodes the unicode string passed in from a RobotFramework test specification.

FYI, my text editor is also configured to strip trailing whitespace when saving a file :-)