target/riscv/kvm.c: add kvmconfig_get_cfg_addr() helper

target/riscv/kvm.c: add kvmconfig_get_cfg_addr() helper

There are 2 places in which we need to get a pointer to a certain
property of the cpu->cfg struct based on property offset. Next patch
will add a couple more.

Create a helper to avoid repeating this code over and over.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-20-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>