libxl: assign a default ssidref (XSM label) to guests

libxl: assign a default ssidref (XSM label) to guests

We have now arranged for SECINITSID_DOMU and SECINITSID_DOMDM to be
defined (correspondng to system_u:system_r:domU_t and
system_u:system_r:dm_dom_t respectively in the default policy). Use
these as the default for the SSID of every (stub)domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Cc: Wei.Liu2@citrix.com
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>