libxl: use libxl__device_kind to get device XS entry

libxl: use libxl__device_kind to get device XS entry

On adding to XS name of device is taken from
libxl__device_kind enum. On getting device from XS
the name is hardcoded. It leads to potential
mistmatch errors. The patch is using libxl__device_kind
everywere to have one source of device name.

Signed-off-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>