Update iface.py to fix the error of Interface name

Sometimes, 'bytes' after 'Interface' are too long, and there is no space between 'Interface' and  'bytes' in /proc/net/dev.
For example, in my /proc/net/dev there is one line begin with 'eth0:137719774170', and the iface will be identified as 'eth0:137719774170'.
![netdev](https://cloud.githubusercontent.com/assets/1490540/4040970/86fa9cd0-2cef-11e4-959d-15e0841a432e.png)