Don't abort if TUNSETSNDBUF fails where no sndbuf= arg is supplied

If no tap,sndbuf= arg is supplied, we use a default value. If
TUNSETSNDBUF fails in this case, we should not abort.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net.c

@@ -1416,7 +1416,7 @@ static void tap_set_sndbuf(TAPState *s, const char *sndbuf_str, Monitor *mon)
         sndbuf = INT_MAX;
     }
 
-    if (ioctl(s->fd, TUNSETSNDBUF, &sndbuf) == -1) {
+    if (ioctl(s->fd, TUNSETSNDBUF, &sndbuf) == -1 && sndbuf_str) {
         config_error(mon, "TUNSETSNDBUF ioctl failed: %s\n",
                      strerror(errno));
     }