target/ricsv: Flush the TLB on virtulisation mode changes

To ensure our TLB isn't out-of-date we flush it on all virt mode
changes. Unlike priv mode this isn't saved in the mmu_idx as all
guests share V=1. The easiest option is just to flush on all changes.

Backports commit eccc5a12c2fd1c646c69a1e7de29183b7a559973 from qemu

qemu/target/riscv/cpu_helper.c

@@ -171,6 +171,11 @@ void riscv_cpu_set_virt_enabled(CPURISCVState *env, bool enable)
         return;
     }
 
+    /* Flush the TLB on all virt mode changes. */
+    if (get_field(env->virt, VIRT_ONOFF) != enable) {
+        tlb_flush(env_cpu(env));
+    }
+
     env->virt = set_field(env->virt, VIRT_ONOFF, enable);
 }