hw/ppc/mac_newworld, mac_oldworld: Don't use load_image()

hw/ppc/mac_newworld, mac_oldworld: 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.
Use the glib g_file_get_contents() function instead, which does
the whole "allocate memory for the file and read it in" operation.

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>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-id: 20181130151712.2312-2-peter.maydell@linaro.org