fix potential warnings with lspv output

See resources/aix/lspv/aix-6.1-hdisk1 for an example of output with
undefined values.

see: #2384
allow easier parameters override, for testing
prevent potential warning with lspv output

And simplifiy parsing by using getFirstMatch() function.

see: #2384
don't forget to use actual command if needed...
more consistency in function names and goals

Keep logical volume info extraction in _getVolumeInfo(), renamed to
_getLogicalVolume() for consistency with similar functions, rename
original _getLogicalVolume() to _getLogicalVolumesFromGroup() and limit
it to returning a list of volume names.

see: #2384
test logical volume names extraction

see: #2384
cosmetics

see: #2384
use a single lsfs call

instead of one by filesystem. It's more efficient, and avoid warnings
when calling it with an unknown filesystem.

see: #2384
merge sample files, drop useless ones
more complete lslv parsing

see: #2384
no need for external cut command
add lsvg parsing test
rename sample files for consistency
run lsvg only once

and extract logical volumes while iterating over volume groups.

closes: #2384