SCSI buses will need to read the children list first-to-last.  This
requires using a QTAILQ, because hell breaks loose if you just try
inserting at the tail (thus reversing the order of all existing
visits from last-to-first to first-to-tail).
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>