target/riscv: fix vs() to return proper error code

vs() should return -RISCV_EXCP_ILLEGAL_INST instead of -1 if rvv feature
is not enabled.

If -1 is returned, exception will be raised and cs->exception_index will
be set to the negative return value. The exception will then be treated
as an instruction access fault instead of illegal instruction fault.

Backports 5e437d3ccdccfd85f6e69ca60f921be2dab62c3c

qemu/target/riscv/csr.c

@@ -61,7 +61,7 @@ static int vs(CPURISCVState *env, int csrno)
     if (env->misa & RVV) {
         return 0;
     }
-    return -1;
+    return -RISCV_EXCP_ILLEGAL_INST;
 }
 
 static int ctr(CPURISCVState *env, int csrno)