docs/system/riscv: update 'virt' machine core limit

docs/system/riscv: update 'virt' machine core limit

The 'virt' RISC-V machine does not have a 8 core limit. The current
limit is set in include/hw/riscv/virt.h, VIRT_CPUS_MAX, set to 512 at
this moment.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1945
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231020200247.334403-2-dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>