vhost-user: add multi queue support

Based on patch by Nikolay Nikolaev:
Vhost-user will implement the multi queue support in a similar way
to what vhost already has - a separate thread for each queue.
To enable the multi queue functionality - a new command line parameter
"queues" is introduced for the vhost-user netdev.

Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>