xbuddy: fix latest-dev keyword crash flash was not able to install image using latest-dev keyword. Listing directory in _LS was not working. BUG=chromium:704174 TEST=Check 'cros flash <ip> xbuddy://remote/eve/latest-dev/test' works. Change-Id: Ic8e9465836308d7897c67861a13527415cd0c2dc Signed-off-by: Gwendal Grignou <gwendal@chromium.org> Reviewed-on: https://chromium-review.googlesource.com/465447 Reviewed-by: Xixuan Wu <xixuan@chromium.org>