Bug#567884: A partial research

[Oren Held]

1. Running grub-probe --device /dev/mapper/myvg-myvol fails (that's THE reason 
for grub-pc postinstall to fail), while grub-probe --device /dev/myvg/myvol 
succeeds (returns "ext2"). I wouldn't expect different behavior on a symlink 
device..

2. The failure shows up in disk/lvm.c grub_lvm_read(). It seems to 
successfully read the LVM entities (Logical Volume, Volume Group), but pv-
>disk is NULL, in the failed cases, for some reason. Didn't figure out why, 
yet, but maybe it'll give a hint to someone..