target/riscv/kvm.c: init 'misa_ext_mask' with scratch CPU

target/riscv/kvm.c: init 'misa_ext_mask' with scratch CPU

At this moment we're retrieving env->misa_ext during
kvm_arch_init_cpu(), leaving env->misa_ext_mask behind.

We want to set env->misa_ext_mask, and we want to set it as early as
possible. The reason is that we're going to use it in the validation
process of the KVM MISA properties we're going to add next. Setting it
during arch_init_cpu() is too late for user validation.

Move the code to a new helper that is going to be called during init()
time, via kvm_riscv_init_user_properties(), like we're already doing for
the machine ID properties. Set both misa_ext and misa_ext_mask to the
same value retrieved by the 'isa' config reg.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-11-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>