target/riscv: fix SBI getchar handler for KVM

Character must be returned via ret[0] field (copied to a0 by KVM).

Return value should be set to 0 to indicate successful processing.

Signed-off-by: Vladimir Isaev <vladimir.isaev@syntacore.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230203135155.12449-1-vladimir.isaev@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/kvm.c

@@ -467,10 +467,11 @@ static int kvm_riscv_handle_sbi(CPUState *cs, struct kvm_run *run)
     case SBI_EXT_0_1_CONSOLE_GETCHAR:
         ret = qemu_chr_fe_read_all(serial_hd(0)->be, &ch, sizeof(ch));
         if (ret == sizeof(ch)) {
-            run->riscv_sbi.args[0] = ch;
+            run->riscv_sbi.ret[0] = ch;
         } else {
-            run->riscv_sbi.args[0] = -1;
+            run->riscv_sbi.ret[0] = -1;
         }
+        ret = 0;
         break;
     default:
         qemu_log_mask(LOG_UNIMP,