libxl: during domain destruction, do not complain if no devices dir to destroy

Previously calling libxl__devices_destroy on a half-constructed or
half-destroyed domain would sometimes complain along these lines:
  libxl: error: libxl_device.c:327:libxl__devices_destroy /local/domain/29/device is empty
This is (a) not a reasonable thing to complain about and (b) not an
accurate description of all the things that that particular failure of
libxl__xs_directory might mean.

Change the code to check errno, so that if errno==ENOENT we silently
continue, not destroying any devices, and if errno!=ENOENT, properly
log the problem and fail.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>

tools/libxl/libxl_device.c

@@ -324,8 +324,12 @@ int libxl__devices_destroy(libxl_ctx *ctx, uint32_t domid, int force)
     path = libxl__sprintf(&gc, "/local/domain/%d/device", domid);
     l1 = libxl__xs_directory(&gc, XBT_NULL, path, &num1);
     if (!l1) {
-        LIBXL__LOG(ctx, LIBXL__LOG_ERROR, "%s is empty", path);
-        goto out;
+        if (errno != ENOENT) {
+            LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_ERROR, "unable to get xenstore"
+                             " device listing %s", path);
+            goto out;
+        }
+        num1 = 0;
     }
     for (i = 0; i < num1; i++) {
         if (!strcmp("vfs", l1[i]))