Do not destroy the device queue, it is needed by the peripheral code

Do not destroy the device queue, it is needed by the peripheral code
(e.g. scsi/scsi_da.c, etc) even if the backend hardware has disappeared.
When cam_sim_free() is called, set up a dummy poll and action callback
and make the action callback generate an I/O error.

This allows things like filesystem mounts of USB memory sticks to start
returning I/O errors instead of blocking forever if the stick is pulled.