vhost: dynamically allocate vhost_worker

This patchset allows us to allocate multiple workers, so this has us
move from the vhost_worker that's embedded in the vhost_dev to
dynamically allocating it.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230626232307.97930-3-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>