lvconvert: disable convertion of thin to mirrors

For now this convertions is not supported, thus disabled.
The only supported conversion for now is to create mirrored thin pools
from mirrored devices.

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.98 -
 =================================
+  Disallow convertion of thins to mirrors.
   Fix lvm2api data_percent reporting for thin volumes.
   Do not allow RAID LVs in a clustered volume group.
   Update lvconvert to support stacking of devs for thin meta/data devs.

tools/lvconvert.c

@@ -1494,6 +1494,13 @@ static int _lvconvert_mirrors(struct cmd_context *cmd,
 		return 0;
 	}
 
+	/* TODO: decide what should be done here */
+	if (lv_is_thin_type(lv)) {
+		log_error("Converting segment type for %s/%s to mirror is not yet supported.",
+			 lv->vg->name, lv->name);
+		return 0;
+	}
+
 	/* Adjust mimage and/or log count */
 	if (!_lvconvert_mirrors_parse_params(cmd, lv, lp,
 					     &old_mimage_count, &old_log_count,