Fixed a bug for ventoy_openwrt.xz check.

INSTALL/grub/grub.cfg

@@ -1740,10 +1740,6 @@ function img_common_menuentry {
         fi
     fi
 
-    if [ -d (vtimghd)/ ]; then
-        loopback -d vtimghd
-    fi
-
     loopback vtimghd "${vtoy_iso_part}${vt_chosen_path}"
     vt_img_sector "${vtoy_iso_part}${vt_chosen_path}"
 
@@ -1808,6 +1804,8 @@ function img_common_menuentry {
         fi
     fi
 
+    loopback -d vtimghd
+
     set root=$vtback_root
     vt_pop_last_entry
     set theme=$vtback_theme