ahci: Use all the cmd slots

AHCI has 32 cmd slots to issue cmd. Only first slot is used currently
which makes the queue-depth 1.

This patch uses all the cmd slots and makes the cmd completion async.
Now, the queue-depth is 32.

Test with "/tests/misc-bdev-write.so" on VBOX shows improvements:

   Before: ~10MB/s
   After: ~20MB/s

1000 round of "/tests/misc-bdev-rw.so" tests passed on VBOX and QEMU.

Signed-off-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>