ringbuf: fix chr_write return value

It should return the number of written bytes.
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022095318.17775-4-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qemu-char.c

@@ -3328,7 +3328,7 @@ static int ringbuf_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
         }
     }
 
-    return 0;
+    return len;
 }
 
 static int ringbuf_chr_read(CharDriverState *chr, uint8_t *buf, int len)