dpif-netdev: Only poll enabled vhost queues.

dpif-netdev: Only poll enabled vhost queues.

We currently poll all available queues based on the max queue count
exchanged with the vhost peer and rely on the vhost library in DPDK to
check the vring status beneath.
This can lead to some overhead when we have a lot of unused queues.

To enhance the situation, we can skip the disabled queues.
On rxq notifications, we make use of the netdev's change_seq number so
that the pmd thread main loop can cache the queue state periodically.

$ ovs-appctl dpif-netdev/pmd-rxq-show
pmd thread numa_id 0 core_id 1:
  isolated : true
  port: dpdk0             queue-id:  0 (enabled)   pmd usage:  0 %
pmd thread numa_id 0 core_id 2:
  isolated : true
  port: vhost1            queue-id:  0 (enabled)   pmd usage:  0 %
  port: vhost3            queue-id:  0 (enabled)   pmd usage:  0 %
pmd thread numa_id 0 core_id 15:
  isolated : true
  port: dpdk1             queue-id:  0 (enabled)   pmd usage:  0 %
pmd thread numa_id 0 core_id 16:
  isolated : true
  port: vhost0            queue-id:  0 (enabled)   pmd usage:  0 %
  port: vhost2            queue-id:  0 (enabled)   pmd usage:  0 %

$ while true; do
  ovs-appctl dpif-netdev/pmd-rxq-show |awk '
  /port: / {
    tot++;
    if ($5 == "(enabled)") {
      en++;
    }
  }
  END {
    print "total: " tot ", enabled: " en
  }'
  sleep 1
done

total: 6, enabled: 2
total: 6, enabled: 2
...

 # Started vm, virtio devices are bound to kernel driver which enables
 # F_MQ + all queue pairs
total: 6, enabled: 2
total: 66, enabled: 66
...

 # Unbound vhost0 and vhost1 from the kernel driver
total: 66, enabled: 66
total: 66, enabled: 34
...

 # Configured kernel bound devices to use only 1 queue pair
total: 66, enabled: 34
total: 66, enabled: 19
total: 66, enabled: 4
...

 # While rebooting the vm
total: 66, enabled: 4
total: 66, enabled: 2
...
total: 66, enabled: 66
...

 # After shutting down the vm
total: 66, enabled: 66
total: 66, enabled: 2

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>