libxl: fix the cleanup of the backend path when using driver domains

With the current libxl implementation the control domain will remove both
the frontend and the backend xenstore paths of a device that's handled by a
driver domain. This is incorrect, since the driver domain possibly needs to
access the backend path in order to perform the disconnection and cleanup of
the device.

Fix this by making sure the control domain only cleans the frontend path,
leaving the backend path to be cleaned by the driver domain. Note that if
the device is not handled by a driver domain the control domain will perform
the removal of both the frontend and the backend paths.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reported-by: Alex Velazquez <alex.j.velazquez@gmail.com>
Cc: Alex Velazquez <alex.j.velazquez@gmail.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
(cherry picked from commit 546678c6a6)

tools/libxl/libxl_device.c

@@ -591,15 +591,15 @@ int libxl__device_destroy(libxl__gc *gc, libxl__device *dev)
 
         if (domid == LIBXL_TOOLSTACK_DOMID) {
             /*
-             * The toolstack domain is in charge for removing both the
-             * frontend and the backend path
+             * The toolstack domain is in charge of removing the
+             * frontend path.
              */
             libxl__xs_path_cleanup(gc, t, fe_path);
-            libxl__xs_path_cleanup(gc, t, be_path);
-        } else if (dev->backend_domid == domid) {
+        }
+        if (dev->backend_domid == domid) {
             /*
-             * The driver domain is in charge for removing what it can
-             * from the backend path
+             * The driver domain is in charge of removing what it can
+             * from the backend path.
              */
             libxl__xs_path_cleanup(gc, t, be_path);
         }