vswitchd: Always configure a default queue for QoS.

When an interface has QoS configured but no default queue (queue 0), OVS
has until now installed that QoS configuration literally, which causes all
packets destined for this default queue to be dropped.  This behavior is
usually both unexpected and undesirable.

This commit changes behavior so that, when no default queue is configured,
OVS uses a default "empty" configuration for the default queue.  This
behavior should be more acceptable when QoS is slightly misconfigured.

I tested that, without this patch, configuring only queue 1 causes "tc
class show" to show only queue 1 (handle 1:2) for linux-htb and linux-hfsc,
and that with this patch it shows configurations for both queue 0 (handle
1:1) and queue 1.

Bug #5583.
Feature #7413.
Signed-off-by: Ben Pfaff <blp@nicira.com>