ide: ahci: reset ncq object to unused on error

When processing NCQ commands, AHCI device emulation prepares a
NCQ transfer object; To which an aio control block(aiocb) object
is assigned in 'execute_ncq_command'. In case, when the NCQ
command is invalid, the 'aiocb' object is not assigned, and NCQ
transfer object is left as 'used'. This leads to a use after
free kind of error in 'bdrv_aio_cancel_async' via 'ahci_reset_port'.
Reset NCQ transfer object to 'unused' to avoid it.

[Maintainer edit: s/ACHI/AHCI/ in the commit message. --js]

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1452282511-4116-1-git-send-email-ppandit@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

hw/ide/ahci.c

@@ -910,6 +910,7 @@ static void ncq_err(NCQTransferState *ncq_tfs)
     ide_state->error = ABRT_ERR;
     ide_state->status = READY_STAT | ERR_STAT;
     ncq_tfs->drive->port_regs.scr_err |= (1 << ncq_tfs->tag);
+    ncq_tfs->used = 0;
 }
 
 static void ncq_finish(NCQTransferState *ncq_tfs)