vhost-scsi: Add support for a worker thread per virtqueue

vhost-scsi: Add support for a worker thread per virtqueue

This adds support for vhost-scsi to be able to create a worker thread
per virtqueue. Right now for vhost-net we get a worker thread per
tx/rx virtqueue pair which scales nicely as we add more virtqueues and
CPUs, but for scsi we get the single worker thread that's shared by all
virtqueues. When trying to send IO to more than 2 virtqueues the single
thread becomes a bottlneck.

This patch adds a new setting, worker_per_virtqueue, which can be set
to:

false: Existing behavior where we get the single worker thread.
true: Create a worker per IO virtqueue.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20231204231618.21962-3-michael.christie@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>