There are two changes in this PR: - I've added `pvFree` to lvmpv format. I need this for LVM cache support in blivet-gui. - Thanks to pvFree we know if the PV is empty, so we can the solve problem with removing empty PVs from VG. We run `pvmove` automatically before `vgreduce` and `pvmove` returns 5 for empty PVs so we think it failed (and 5 is not unique return code for this situation).