Add --name= to ksdata for btrfs subvols, where we can.

This only works for volumes created by anaconda.  Pre-existing volumes may
have been created without a label, which means we don't have any definite
way of grabbing the parent of the subvol.  In those cases, we'll log it
so the traceback will at least have some sign of what went wrong.

Resolves: rhbz#1072060