hw/riscv/virt: Remove unnecessary use of &first_cpu

hw/riscv/virt: Remove unnecessary use of &first_cpu

virt_machine_init() creates the HARTs vCPUs, then later
virt_machine_done() calls create_fdt_sockets(), so the
latter has access to the first vCPU via:

  RISCVVirtState {
    RISCVHartArrayState {
      RISCVCPU *harts;
      ...

    } soc[VIRT_SOCKETS_MAX];
    ...

  } s;

Directly use that instead of the &first_cpu global.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250112231344.34632-3-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>