classes/fs-uuid: Fix command output decoding issue

The default return value from subprocess.check_output is an encoded byte.
The applied fix will decode the value to a string.

Signed-off-by: Pawel Zalewski <pzalewski@thegoodpenguin.co.uk>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 046769fa)
Signed-off-by: Steve Sakoman <steve@sakoman.com>