Allow Partition > New on unallocated whole disk devices again (#788308)

Fix up following switch from whole_device flag to TYPE_UNPARTITIONED.

Again use FS_UNALLOCATED to determine if a partition represents
unallocated space and creation of a new partition should be allowed.
This is so that trying to create a new partition on a whole disk device
shows the "No partition table found on device /dev/DEV" warning again.

Bug 788308 - Remove whole_device partition flag