virtio: document ENOSPC

drivers handle ENOSPC specially since it's an error one can
get from a working VQ. Document the semantics.

Message-Id: <2e6ec46b8d5e6755be291cec8e2ec57ef286e97b.1748356035.git.mst@redhat.com>
Reported-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>