Use shorter label for images

dotfiles/i3status.conf

@@ -54,7 +54,7 @@ tztime local {
 }
 
 disk "/var/lib/libvirt/images" {
-    format = "virt:%avail"
+    format = "i:%avail"
     threshold_type = "gbytes_avail"
     low_threshold = 2
 }