Fix backend.GetVolumeList handling of LVM output

Sometimes ‘lvs’ can spit error messages on stdout, even when one wants
to parse the output:
...
Inconsistent metadata copies found - updating to use version 2776
...

So we need to validate the output to guard against such cases.

The patch converts the split on the separater to match against a regex
and extract the fields via groups. The original separator choice is a
bad one now :(

Reviewed-by: imsnah