hw/riscv/virt.c: change 'aclint' TCG check

The 'aclint' property is being conditioned with tcg acceleration in
virt_machine_class_init(). But acceleration code starts later than the
class init of the board, meaning that tcg_enabled() will be always be
false during class_init(), and the option is never being declared even
when declaring TCG accel:

$ ./build/qemu-system-riscv64 -M virt,accel=tcg,aclint=on
qemu-system-riscv64: Property 'virt-machine.aclint' not found

Fix it by moving the check from class_init() to machine_init(). Tune the
description to mention that the option is TCG only.

Cc: Philippe Mathieu-Daudé <philmd@linaro.org>
Fixes: c0716c81 ("hw/riscv/virt: Restrict ACLINT to TCG")
Resolves: qemu-project/qemu#1823


Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20230811160224.440697-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>