Cleanup fid finalization code in free_vg and allow exactly the same fid to be set...

Cleanup fid finalization code in free_vg and allow exactly the same fid to be set again for a PV/VG.

Actually, we can call vg_set_fid(vg, NULL) instead of calling
destroy_instance for all PV structs and a VG struct - it's the same
code we already have in the vg_set_fid.

Also, allow exactly the same fid to be set again for the same PV/VG
Before, this could end up with the fid destroyed because we destroyed
existing fid first and then we used the new one and we didn't care
whether existing one == new one by chance.