scsi-disk: lazily allocate bounce buffer

It will not be needed for reads and writes if the HBA provides a sglist.
In addition, this lets scsi-disk refuse commands with an excessive
allocation length, as well as limit memory on usual well-behaved guests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
static void scsi_destroy(SCSIDevice *dev)
{
    SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, dev);

    scsi_device_purge_requests(&s->qdev, SENSE_CODE(NO_SENSE));
    blockdev_mark_auto_del(s->qdev.conf.bs);
}

static void scsi_destroy(SCSIDevice *dev)
{
    SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, dev);

    scsi_device_purge_requests(&s->qdev, SENSE_CODE(NO_SENSE));
    blockdev_mark_auto_del(s->qdev.conf.bs);
}
