RISC-V: KVM: Use nacl_csr_xyz() for accessing CSRs in run-loop

When running under some other hypervisor, we should use nacl_csr_xyz()
for accessing most CSRs in run-loop. This makes CSR access faster
whenever SBI nested acceleration is available.

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