char: translate from QIOChannel error to errno

Caller of CharDriverState.chr* callback assume errno error conventions.
Translate QIOChannel error to errno (this fixes potential EAGAIN
regression, for ex if a vhost-user backend block, qemu_chr_fe_read_all()
could get error -2 and not wait)

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1457718924-19338-1-git-send-email-marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

qemu-char.c

@@ -2799,6 +2799,13 @@ static ssize_t tcp_chr_recv(CharDriverState *chr, char *buf, size_t len)
                                      NULL);
     }
 
+    if (ret == QIO_CHANNEL_ERR_BLOCK) {
+        errno = EAGAIN;
+        ret = -1;
+    } else if (ret == -1) {
+        errno = EIO;
+    }
+
     if (msgfds_num) {
         /* close and clean read_msgfds */
         for (i = 0; i < s->read_msgfds_num; i++) {