FileUtils: add method to get UUID of a volume

Summary: Change-Id: I41b76c5e01c82a2f8e67b33133a76947877a2814

Test Plan: Build and test.

Reviewers: Mustaavalkosta, randall.rushing, michyprima, rlasalle15, Roach, synergy

Differential Revision: http://review.codefirex.com/D260