ahci: Fix crashes on duplicate BH registration

If ahci_dma_set_inactive is called a while there is still a pending BH
from a previous run, we will crash on the second run of
ahci_check_cmd_bh as it overwrites AHCIDevice::check_bh. Avoid this
broken and redundant duplicate registration.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/ide/ahci.c

@@ -1066,9 +1066,11 @@ static int ahci_dma_set_inactive(IDEDMA *dma)
 
     ad->dma_cb = NULL;
 
+    if (!ad->check_bh) {
         /* maybe we still have something to process, check later */
         ad->check_bh = qemu_bh_new(ahci_check_cmd_bh, ad);
         qemu_bh_schedule(ad->check_bh);
+    }
 
     return 0;
 }