RISCV: KVM: Add senvcfg context save/restore

Add senvcfg context save/restore for guest VCPUs and also add it to the
ONE_REG interface to allow its access from user space.

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