RISC-V: KVM: Use NACL HFENCEs for KVM request based HFENCEs

When running under some other hypervisor, we should use NACL HFENCEs
for TLB shoot-down via KVM requests. This makes HFENCEs faster whenever
SBI nested acceleration is available.

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