A protected partition may be shown in custom, but it should not be
possible to remove it. This makes sure that the remove button is set to
the mountpoint's state when the page header is clicked.
Make sure the 'delete all' on other partitions of the disk don't delete
the protected partition by checking in _destroy_device, also exit early
if there was a failure, it doesn't seem like a good idea to continue
with the rest of the method if there was an error.
Also make sure the protected device cannot be resized by making the
_sizeEntry insensitive.
Resolves: rhbz#1052883
Looks alright to me.
- Chris