RISC-V: KVM: Save/restore HSTATUS in C source

We will be optimizing HSTATUS CSR access via shared memory setup
using SBI nested acceleration extension. To facilitate this, we
first move HSTATUS save/restore in kvm_riscv_vcpu_enter_exit().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>