Send all the SVQ control commands in parallel instead of serialized
Goal
All CVQ state load commands are sent serially by the CVQ. These include the load of mac address or multiqueue status.
This is not optimal when a lot of state needs to be restored at the destination of a migration. Virtqueues already allows for the restore commands to be queued and wait for all of them to be completed in parallel.
Technical details
All the changes should start from vhost_vdpa_net_load.
The specific CVQ commands like MAC and MQ are sent and actively waited from individual functions. The active waiting can be maintained, but we should return the number of used buffers so we don't miss any of them and we don't wait for buffers never sent.