Description of problem:

For testing purposed it might be occasionally useful to play with LVs that have 'zero' or 'error' segtype. 

Possible usecases:

lvcreate -L1000G --type zero -n lvname  vgname

lvconvert --thinpool vgname/lvname

or

lvconvert --thin  vgname/lvname

lvconvert --cachepool vgname/lvname


Allows to quickly create a thin volume with any size - that has *NO REAL* space used for backend - so clearly not usable to story *ANY REAL* data - but allow to check various performance characteristics of thin  or cache targets.