iscsi: fix races between task completion and abort

This patch fixes two main issues with block/iscsi.c:

1) iscsi_task_mgmt_abort_task_async calls iscsi_scsi_task_cancel which
was also directly called in iscsi_aio_cancel

2) a race between task completion and task abortion could happen cause
the scsi_free_scsi_task were done before iscsi_schedule_bh has finished.
To fix this, all the freeing of IscsiTasks and releasing of the AIOCBs
is centralized in iscsi_bh_cb, independent of whether the SCSI command
has completed or was cancelled.

3) iscsi_aio_cancel was not synchronously waiting for the end of the
command.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
