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

hw/i386/pc.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.
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 <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-id: 20181130151712[email protected]