Close up a rare multipath issue.

Close up small hole where a command completes after a device has been
removed from SML and before the device is re-added.

 - Mark device as removed in slave_destroy

 - Do not complete commands for deleted devices
Reviewed-by: Justin Lindley <justin.lindley@microsemi.com>
Reviewed-by: David Carroll <david.carroll@microsemi.com>
Reviewed-by: Scott Teel <scott.teel@microsemi.com>
Signed-off-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>