Bug#768897: MBR disklabels also yield destructive pvcreate

[Drake Wilson <drake@dasyatidae.net>]

FYI: I've just confirmed with partman-lvm 99 (plus whatever libparted is in
the last Debian testing weekly ISO) that MBR disklabels using 8e (Linux LVM)
as a type code for LUKS are also affected by this.  So it's not just GPT.
It's arguably even more dangerous for MBR, because the type code space is
so small that collisions should be expected, but util-linux's fdisk in MBR
mode also provides a 0xda code for "non-FS data", so users in that case may
be less tempted to default to the underlying volume type.

   ---> Drake Wilson