tests/qemu-iotests: Fix check for existing file in _require_disk_usage()

Looks like the "$" has been forgotten here to get the contents of
the FILENAME variable.

Fixes: c49dda7254 ("iotests: Filter out ZFS in several tests")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20251208075320.35682-1-thuth@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/common.rc

@@ -1008,7 +1008,7 @@ _require_disk_usage()
     else
         FILENAME="$TEST_IMG_FILE"
     fi
-    if [ -e "FILENAME" ]; then
+    if [ -e "$FILENAME" ]; then
         echo "unwilling to overwrite existing file"
         exit 1
     fi