hw/core: properly terminate loading .hex on EOF record

hw/core: properly terminate loading .hex on EOF record

The https://makecode.microbit.org/#editor generates slightly weird
.hex files which work fine on a real microbit but causes QEMU to
choke. The reason is extraneous data after the EOF record which causes
the loader to attempt to write a bigger file than it should to the
"rom". According to the HEX file spec an EOF really should be the last
thing we process so lets do that.

Reported-by: Ursula Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200403191150.863-12-alex.bennee@linaro.org>