VfatDiskMount: fix vfat UUID string

VfatDiskMount: fix vfat UUID string

The right format for the UUID string for vfat in Linux is 'XXXX-XXXX'. This is
how it can be used in fstab, for example (UUID='XXXX-XXXX'). Also, Linux udev
creates /dev/disk/by-uuid/XXXX-XXXX entries for vfat volumes.

The problem in VfatDiskMount is that it provides it in the 'XXXXXXXX' form
(without dash). However, mkfs.vfat does not like the dash, and this is why we
did not have it, I guess.

This patch fixes the situation and makes VfatDiskMount class expose UUID in a
fstab-friendly format. This makes --fstab-entry=uuid option also work for VFAT
volumes (I tested this).

Change-Id: I1d676e3707ef1777df910273381fe4437b415f41
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>