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

We extend the KVM ISA extension ONE_REG interface to allow KVM
user space to detect and enable Zfa 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>