RISC-V: KVM: Add kvm_vcpu_config

Add a placeholder for all registers such as henvcfg, hstateen etc
which have 'static' configurations depending on extensions supported by
the guest. The values are derived once and are then subsequently written
to the corresponding CSRs while switching to the vcpu.

Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>