libata creates one thread per ata_port structure.  This is inadequate
for our needs, and also cumbersome to maintain, now that workqueues
and Rusty's thread work is available.

This patch begins to move libata away from doing its own per-port
thread, by moving the ATAPI command initiation code to work under
the workqueue system.

This patch also creates a private workqueue, global to all of libata.