Replace free_vg with release_vg

Move the free_vg() to  vg.c  and replace free_vg  with release_vg
and make the _free_vg internal.

Patch is needed for sharing VG in vginfo cache so the release_vg function name
is a better fit here.

lib/format_pool/format_pool.c

@@ -156,7 +156,7 @@ static struct volume_group *_pool_vg_read(struct format_instance *fid,
 	return vg;
 
 bad:
-	free_vg(vg);
+	release_vg(vg);
 
 	return NULL;
 }