Select unallocated whole disk devices again (#788308)

After the change from whole_device flag to TYPE_UNPARTITIONED,
unallocated whole disk devices are no longer automatically selected
because the partition type is no longer TYPE_UNALLOCATED.  Fix by
checking for file system type FS_UNALLOCATED when identifying the
largest unallocated space.

Bug 788308 - Remove whole_device partition flag