fix bug setting device

The device set for the storage mount point was
hard-coded to /dev/$name. This value is only
valid when no device is supplied to the storage
xfs setup.

This commit updates it to use the target_device
which is either the device that was passed, or
/dev/$name if not device is specified.
Merge pull request #84 from bodepd/swift_fix_xfs_mount_device

fix bug setting device