hw/riscv: virt: Allow support for 32 cores

hw/riscv: virt: Allow support for 32 cores

Linux supports up to 32 cores for both 32-bit and 64-bit RISC-V, so
let's set that as the maximum for the virt board.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/435
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-Id: <20220105213937.1113508-9-alistair.francis@opensource.wdc.com>