virtio-scsi: avoid cancelling uninitialized work items

Calling the workqueue interface on uninitialized work items isn't a
good idea even if they're zeroed. It's not failing catastrophically only
through happy accidents.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>