x86/tdx: Mark message.bytes as nonstring

In preparation for strtomem*() checking that its destination is a
nonstring, rename and annotate message.bytes accordingly.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Kees Cook <kees@kernel.org>