target/riscv/kvm: change kvm_reg_id to uint64_t

The field isn't big enough to hold an uint64_t kvm register and Vector
registers will end up overflowing it.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240123161714.160149-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>