nbd: Fix nbd_establish_connection()'s return value

unix_connect_opts() and inet_connect_opts() do not necessarily set errno
(if at all); therefore, nbd_establish_connection() should not literally
return -errno on error.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-Id: <1424887718-10800-4-git-send-email-mreitz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

block/nbd.c

@@ -248,7 +248,7 @@ static int nbd_establish_connection(BlockDriverState *bs, Error **errp)
     /* Failed to establish connection */
     if (sock < 0) {
         logout("Failed to establish connection to NBD server\n");
-        return -errno;
+        return -EIO;
     }
 
     return sock;