vdpa_sim: use kthread worker

Let's use our own kthread to run device jobs.
This allows us more flexibility, especially we can attach the kthread
to the user address space when vDPA uses user's VA.

Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20230404131725.45908-1-sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>