libxenlight: fix memory leaks

libxenlight: fix memory leaks

In particular:

- all the temporary flexarrays allocated in the create
  device functions must be freed;

- all the strings that don't need to be modified can be added as they
  are
  to these temporary flexarrays instead of duplicating them;

- any data returned to the user shouldn't be added to the global
  memory tracker so that the user can free it whenever he wishes.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>