Description of problem:
Can't boot from the system img when boot guest with  38 virtio-blk-pci disks via pci-bridge, if boot with 18,35,36,37 virtio-blk-pci disks via pci-bridge work well

Preparation:
Exist system IMG:/home/RHEL-Server-7.0-64.qcow2

#for((i=1;i<40;i++));do qemu-img create -f qcow2 /home/disk$i 100M;done

Version-Release number of selected component (if applicable):
Host:
]# uname -r
3.10.0-86.el7.x86_64
# rpm -q qemu-kvm
qemu-kvm-1.5.3-48.el7.x86_64
# rpm -q seabios
seabios-1.7.2.2-11.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Boot guest with shell script max_bridge_device.sh.
#sh max_bridge_device.sh  38
...
# cat max_bridge_device.sh 
#!/bin/sh

MACHINE=pc-i440fx-rhel7.0.0
MEM=4G
IMG=/home/RHEL-Server-7.0-64.qcow2

CLI="/usr/libexec/qemu-kvm -vga none -net none -M $MACHINE -smp 4,core=2,thread=2,socket=1 -m $MEM -name vm1 -vnc :1 -vga qxl -monitor stdio -device pci-bridge,chassis_nr=1,id=bridge0,addr=0x08 -drive file=$IMG,if=none,id=hd,format=qcow2,werror=stop,rerror=stop -device virtio-blk-pci,scsi=off,drive=hd,id=os-disk,bus=bridge0,addr=0x01,bootindex=1 -serial unix:/tmp/bridge-con,server,nowait -boot menu=on,reboot-timeout=10 -monitor stdio"

while [ ${i:=1} -lt ${1:-0} ]
do
    createBr=$((i%31))
    bridgeId=$((i/31))
    slotId=$((i%31+1))
    if [ $createBr -eq 0 ]
    then
        CLI="$CLI -device pci-bridge,chassis_nr=$((bridgeId+1)),id=bridge$bridgeId,addr=0x$(printf "%02x" $((bridgeId+2)))"
    fi
    CLI="$CLI -drive file=/home/disk/disk$i,if=none,id=disk$i"
    CLI="$CLI -device virtio-blk-pci,bus=bridge$bridgeId,drive=disk$i,id=blk$i,addr=0x$(printf "%02x" $slotId)"
    ((i++))
done

$CLI

2.Press F12,will show 7 virtio disks on the seabios boot menu, this is correct(see bug1034072 C#9)--->see attachment
 
3.Press 1,try to boot the guest

Actual results:
Can't boot from the system IMG. will show
"Boot failed:not a bootable disk" 

Expected results:
Can boot from the system IMG

Additional info: