vdpa_sim: Implement resume vdpa op

Implement resume operation for vdpa_sim devices, so vhost-vdpa will
offer that backend feature and userspace can effectively resume the
device.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Message-Id: <15a4566826033c5dd9a2167e5cfb0ef4d90cea49.1672742878.git.sebastien.boeuf@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>