gpo: Properly decode utf-8/16 inf files from bytes

gpo: Properly decode utf-8/16 inf files from bytes

This code was python 2 specific (string handling
has changed dramatically in python 3), and didn't
correctly decode utf-16 in python3. We should
instead read the file as bytes, then attempt a
utf-8 decode (the default), and try utf-16 if
encountering a decode failure.
The existing code actually throws an exception on
the initial file read when the data is utf-16,
since it tries to decode the bytes to a utf-8
string.

Signed-off-by: David Mulder <dmulder@suse.com>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>