lvconvert: thin conversion accept passed PVs

lvconvert: thin conversion accept passed PVs

When creating thin pool or check pool there is allocated LV
for metadata and for such allocation user should be able to
specify list of PVs on cmdline.

Also fix unused passed list of PV for thick to thin conversion,
where the code was using whole PV set from a VG (but since it's
been not enabled on cmdline, user could not hit this issue).
Also remove unneeded initialization of use_pvh.