lvconvert: fix vg parameter

lvconvert: fix vg parameter

Since we want to read env LVM_VG_NAME vg names,
we cannot just check LV names which do contain '/'.

So before the patch commands like:

> lvconvert --repair vg

Before:

Please provide a valid volume group name

After:
Path required for Logical Volume "vg".
Please provide a valid volume group name

> LVM_VG_NAME=vg lvconvert --repair vg

Before:
Please provide a valid volume group name

After:
Can't find LV vg in VG vg