Correct whole disk device file system format to cleared preview (#743181)

The preview of clearing a whole disk device file system was previewing
the same as formatting to all other file system types; as a cleared file
system spanning the whole disk device.  However when implemented this
removes all signatures on the disk so it actually becomes an unallocated
and unpartitioned device.  Make the preview match what happens in when
implemented.

GParted previously used mydevice.max_prims = -1 to represent an
unpartitioned device.  It is now represented as:

    mydevice.max_prims = 1
    mydevice.disktype  = _("unrecognized")
    mydevice.partitions[0].type         = TYPE_UNALLOCATED
    mydevice.partitions[0].whole_device = true
    mydevice.partitions[0].filesystem   = FS_UNALLOCATED

and the check for an unpartitioned device in Win_GParted.cc becomes:

    partitions[0].type == TYPE_UNALLOCATED && partitions[0].whole_device

Bug 743181 - Add unpartitioned drive read-write support