libxl_device_vtpm_getinfo needs to examine devices without trusting
frontend-controlled data. So:
* Use /libxl to find the backend path.
* Parse the backend path to find the backend domid, rather than
reading it from the frontend.
This is part of XSA-175.
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>