hw/char/sifive_uart: Free fifo on unrealize

hw/char/sifive_uart: Free fifo on unrealize

We previously allocate the fifo on reset and never free it, which means
we are leaking memory.

Instead let's allocate on realize and free on unrealize.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Tested-by: Clément Chigot <chigot@adacore.com>
Message-ID: <20250303023120.157221-1-alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>