libxl: correctly dispose of dominfo list in libxl_name_to_domid

Tamas reported ssid_label was leaked. Use the designated function to
free dominfo list to fix the leakage.

Reported-by: Tamas K Lengyel <tamas@tklengyel.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>
(cherry picked from commit f50dd67950)

tools/libxl/libxl_utils.c

@@ -90,7 +90,7 @@ int libxl_name_to_domid(libxl_ctx *ctx, const char *name,
         }
         free(domname);
     }
-    free(dominfo);
+    libxl_dominfo_list_free(dominfo, nb_domains);
     return ret;
 }