util/hexdump: Reorder qemu_hexdump() arguments

qemu_hexdump()'s pointer to the buffer and length of the
buffer are closely related arguments but are widely separated
in the argument list order (also, the format of <stdio.h>
function prototypes is usually to have the FILE* argument
coming first).

Reorder the arguments as "fp, prefix, buf, size" which is
more logical.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200822180950.1343963-3-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>