libxl: Do not trust backend for disk in getinfo

Do not read the frontend path out of the backend.  We have it in our
hand.  Likewise the guest (frontend) domid was one of our parameters (!)

This is part of XSA-178.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl.c

@@ -2440,9 +2440,8 @@ int libxl_device_disk_getinfo(libxl_ctx *ctx, uint32_t domid,
     val = libxl__xs_read(gc, XBT_NULL, GCSPRINTF("%s/ring-ref", fe_path));
     diskinfo->rref = val ? strtoul(val, NULL, 10) : -1;
     diskinfo->frontend = xs_read(ctx->xsh, XBT_NULL,
-                                 libxl__sprintf(gc, "%s/frontend", diskinfo->backend), NULL);
-    val = libxl__xs_read(gc, XBT_NULL, libxl__sprintf(gc, "%s/frontend-id", diskinfo->backend));
-    diskinfo->frontend_id = val ? strtoul(val, NULL, 10) : -1;
+                                 GCSPRINTF("%s/frontend", libxl_path), NULL);
+    diskinfo->frontend_id = domid;
 
     GC_FREE;
     return 0;