hw/riscv/virt.c: remove trivial virt_memmap references

We should use s->memmap instead of virt_memmap to be able to use an
updated memmap when we start versioning the board.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250429125811.224803-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>