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

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: c49dda7254d ("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>