hw/riscv/boot.c: use MachineState in riscv_load_kernel()

hw/riscv/boot.c: use MachineState in riscv_load_kernel()

All callers are using kernel_filename as machine->kernel_filename.

This will also simplify the changes in riscv_load_kernel() that we're
going to do next.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230102115241.25733-10-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>