9p: local: Properly set errp in fstatfs error path

In the review of

    9p: Avoid warning if FS_IOC_GETVERSION is not defined

Grep Kurz noted this error path was failing to set errp.
Fix that.
Signed-off-by: NKeno Fischer <keno@juliacomputing.com>
[added local: to commit title, Greg Kurz]
Signed-off-by: NGreg Kurz <groug@kaod.org>

hw/9pfs/9p-local.c

@@ -1418,6 +1418,8 @@ static int local_init(FsContext *ctx, Error **errp)
      */
     if (fstatfs(data->mountfd, &stbuf) < 0) {
         close_preserve_errno(data->mountfd);
+        error_setg_errno(errp, errno,
+            "failed to stat file system at '%s'", ctx->fs_root);
         goto err;
     }
     switch (stbuf.f_type) {