xl: destroy the disk on successful removal during block-detach

main_blockdetach needs to call libxl_device_disk_destroy so that all the
disk related entries are properly removed from xenstore.

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

tools/libxl/xl_cmdimpl.c

@@ -5405,7 +5405,8 @@ int main_blockdetach(int argc, char **argv)
     }
     if (libxl_device_disk_remove(ctx, domid, &disk, 0)) {
         fprintf(stderr, "libxl_device_disk_remove failed.\n");
-    }
+    } else
+        libxl_device_disk_destroy(ctx, domid, &disk);
     return 0;
 }