lvconvert: fixing extraction of vgname

lvconvert: fixing extraction of vgname

Correction to function for extracting vgname out of lvconvert
parameters.

Avoid repeating some checks.

Add code to handle generic options which may provide vgname in its argument
and compare them all so they match to a single vgname (otherwise it's a
error).

Extract default (envvar) vgname only when no position nor optional vgname is
found.

Fixing regression instroduce with patchset started with commit:
1e2420bca85da9a37570871cd70192e9ae831786   (2.02.169)