hw/core: Make do_unaligned_access noreturn

hw/core: Make do_unaligned_access noreturn

While we may have had some thought of allowing system-mode
to return from this hook, we have no guests that require this.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>