Internally, qemu may create chardev without ID. Those will not be
looked up with qemu_chr_find(), which prevents using qdev_prop_set_chr().

Use id_generate(), to generate an internal name (prefixed with #), so
no conflict exist with user-named chardev.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: xiaoqiang zhao <zxq_yx_007@163.com>