util/hexdump: Convert to take a void pointer argument

Most uses of qemu_hexdump() do not take an array of char
as input, forcing use of cast. Since we can use this
helper to dump any kind of buffer, use a pointer to void
argument instead.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200822180950.1343963-2-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>