lvconvert: add segtypes raid6_{ls,rs,la,ra}_6 and conversions to/from it

lvconvert: add segtypes raid6_{ls,rs,la,ra}_6 and conversions to/from it

Add:
- support for segment types raid6_{ls,rs,la,ra}_6
  (striped raid with dedicated last Q-Syndrome SubLVs)
- conversion support from raid5_{ls,rs,la,ra} to/from raid6_{ls,rs,la,ra}_6
- setting convenient segtypes on conversions from/to raid4/5/6
- related tests to lvconvert-raid-takeover.sh factoring
  out _lvcreate,_lvconvert funxtions

Related: rhbz1366296