9pfs: fix information leak in xattr read

9pfs uses g_malloc() to allocate the xattr memory space, if the guest
reads this memory before writing to it, this will leak host heap memory
to the guest. This patch avoid this.
Signed-off-by: NLi Qiang <liqiang6-s@360.cn>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NGreg Kurz <groug@kaod.org>

hw/9pfs/9p.c

@@ -3282,7 +3282,7 @@ static void coroutine_fn v9fs_xattrcreate(void *opaque)
     xattr_fidp->fs.xattr.flags = flags;
     v9fs_string_init(&xattr_fidp->fs.xattr.name);
     v9fs_string_copy(&xattr_fidp->fs.xattr.name, &name);
-    xattr_fidp->fs.xattr.value = g_malloc(size);
+    xattr_fidp->fs.xattr.value = g_malloc0(size);
     err = offset;
     put_fid(pdu, file_fidp);
 out_nofid: