RISC-V: KVM: Allow Zacas extension for Guest/VM

Extend the KVM ISA extension ONE_REG interface to allow KVM user space
to detect and enable Zacas extension for Guest/VM.

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