Revert "dt: temporarily disable subtree creation failure check"

This reverts commit "dt: temporarily disable subtree creation
failure check" which was meant as a temporary solution to keep
external and dynamic device tree construction intact.

Now that we switched to fully dynamic dt construction, it's no
longer necessary.

Signed-off-by: Alexander Graf <agraf@suse.de>

device_tree.c

@@ -239,13 +239,11 @@ int qemu_devtree_add_subnode(void *fdt, const char *name)
     }
 
     retval = fdt_add_subnode(fdt, parent, basename);
-#if 0
     if (retval < 0) {
         fprintf(stderr, "FDT: Failed to create subnode %s: %s\n", name,
                 fdt_strerror(retval));
         exit(1);
     }
-#endif
 
     g_free(dupname);
     return retval;