libxl: do not assume Dom0 backend while getting nic info

Fill backend_domid field based on backend path.

[ Correspoding xen.git#master commit is 7539772a65 -iwj ]

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
(cherry picked from commit 317eb71bc3)

tools/libxl/libxl.c

@@ -3485,6 +3485,18 @@ static int libxl__device_nic_from_xenstore(libxl__gc *gc,
     else
         nic->devid = 0;
 
+    rc = libxl__xs_read_checked(gc, XBT_NULL,
+                                GCSPRINTF("%s/backend", libxl_path), &tmp);
+    if (rc) goto out;
+
+    if (!tmp) {
+        LOG(ERROR, "nic %s does not exist (no backend path)", libxl_path);
+        rc = ERROR_FAIL;
+        goto out;
+    }
+    rc = libxl__backendpath_parse_domid(gc, tmp, &nic->backend_domid);
+    if (rc) goto out;
+
     /* nic->mtu = */
 
     tmp = READ_LIBXLDEV(gc, "mac");