libxl: Do not trust backend for vtpm in getinfo (uuid)

libxl: Do not trust backend for vtpm in getinfo (uuid)

Use uuid from /libxl, rather than from backend.  I think the backend
is not supposed to change the uuid, since it seems to be set by libxl
during setup.

If in fact the backend is supposed to be able to change the uuid, this
patch needs to be dropped and replaced by a patch which makes the vtpm
uuid lookup tolerate bad or missing data.

This is part of XSA-178.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>