hw/i386/multiboot.c: Don't use load_image()

hw/i386/multiboot.c: Don't use load_image()

The load_image() function is deprecated, as it does not let the
caller specify how large the buffer to read the file into is.
Instead use load_image_size().

While we are converting the code, add the missing error check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20181130151712.2312-7-peter.maydell@linaro.org