libxl: Only call stat() when adding a disk if we expect a device to exist.

We consider calling stat() a helpful error check in the following
circumstances only:
 1. the disk backend type must be PHYsical
 2. the disk backend domain must be the same as the running libxl
    code (ie LIBXL_TOOLSTACK_DOMID)
 3. there must not be a hotplug script because this would imply that
    the device won't be created until after the hotplug script has
    run.

With this fix, it is possible to use qemu's built-in block drivers
such as ceph/rbd, with a xl config disk spec like this:

disk=[ 'backendtype=qdisk,format=raw,vdev=hda,access=rw,target=rbd:rbd/ubuntu1204.img' ]

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

tools/libxl/libxl_device.c

@@ -236,7 +236,9 @@ int libxl__device_disk_set_backend(libxl__gc *gc, libxl_device_disk *disk) {
             return ERROR_INVAL;
         }
         memset(&a.stab, 0, sizeof(a.stab));
-    } else if (!disk->script) {
+    } else if (disk->backend == LIBXL_DISK_BACKEND_PHY &&
+               disk->backend_domid == LIBXL_TOOLSTACK_DOMID &&
+               !disk->script) {
         if (stat(disk->pdev_path, &a.stab)) {
             LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_ERROR, "Disk vdev=%s "
                              "failed to stat: %s",