Fix double quote error creating XFS file systems (#744108)

Trying to create an XFS file system causes the following error and
the create new xfs file system step to never complete.

    # ./gpartedbin
    ======================
    libparted : 2.4
    ======================

    (gpartedbin:15572): glibmm-CRITICAL **:
    unhandled exception (type Glib::Error) in signal handler:
    domain: g-shell-error-quark
    code  : 0
    what  : Text ended before matching quote was found for ". (The text was 'mkfs.xfs -f -L "mylabel" " /dev/sdb8')

Creation of XFS file systems was broken by the extra stray double quote
added to the command before the device name, by commit:

    63aeb150
    Rename member variables and methods in Partition class (#741424)

Bug 744108 - Creating an XFS file system never completes