Fix per-vg checks in instance disk conversion

Since we now have just two possibilities (plaint to drbd or drbd to
plain), we can assume (+an assert) that we come from LVM and access
directly logical_id[0] for the VG on which it is stored.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>