The following patches allow users to configure the vhost worker threads
for vhost-scsi. With vhost-net we get a worker thread per rx/tx virtqueue
pair, but for vhost-scsi we get one worker for all workqueues. This
becomes a bottlneck after 2 queues are used.

In the upstream linux kernel commit:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/vhost/vhost.c?id=c1ecd8e9500797748ae4f79657971955d452d69d

we enabled the vhost layer to be able to create a worker thread and
attach it to a virtqueue.

This patchset adds support to vhost-scsi to use these ioctls so we are
no longer limited to the single worker.