Description of problem:
 Failed to boot with multi-disks when the first disk is not bootable

Version-Release number of selected component (if applicable):
kernel version::3.10.0-742.el7.x86_64
qemu-kvm-rhev version: qemu-kvm-rhev-2.10.0-3.el7.x86_64
seabios version: seabios-bin-1.10.2-5.el7.noarch


How reproducible:
100%

Steps to Reproduce:
1.Create img "usbdevice.qcow2" with no OS installed on it.
2.Start guest with qemu cmds:
  /usr/libexec/qemu-kvm \
    -name 'avocado-vt-vm1'  \
    -sandbox off  \
    -machine pc  \
    -nodefaults  \
    -vga cirrus  \
    -chardev socket,id=qmp_id_qmpmonitor1,path=/var/tmp/monitor-qmpmonitor1-20171022-043431-Oqzhk5Cb,server,nowait \
    -mon chardev=qmp_id_qmpmonitor1,mode=control  \
    -chardev socket,id=qmp_id_catch_monitor,path=/var/tmp/monitor-catch_monitor-20171022-043431-Oqzhk5Cb,server,nowait \
    -mon chardev=qmp_id_catch_monitor,mode=control \
    -device pvpanic,ioport=0x505,id=idFEWDmx  \
    -chardev socket,id=serial_id_serial0,path=/var/tmp/serial-serial0-20171022-043431-Oqzhk5Cb,server,nowait \
    -device isa-serial,chardev=serial_id_serial0  \
    -chardev socket,id=seabioslog_id_20171022-043431-Oqzhk5Cb,path=/var/tmp/seabios-20171022-043431-Oqzhk5Cb,server,nowait \
    -device isa-debugcon,chardev=seabioslog_id_20171022-043431-Oqzhk5Cb,iobase=0x402 \
    -device usb-ehci,id=usb1,bus=pci.0,addr=0x3 \
    -drive id=drive_image1,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/rhel75-64-virtio.qcow2 \
    -device ide-hd,id=image1,drive=drive_image1,bootindex=1,bus=ide.0,unit=0 \
    -drive id=drive_stg,if=none,snapshot=off,aio=native,cache=none,format=qcow2,file=/home/kvm_autotest_root/images/usbdevice.qcow2 \
    -device usb-storage,id=stg,drive=drive_stg,bootindex=0,bus=usb1.0,port=1 \
    -device virtio-net-pci,mac=9a:d2:d3:d4:d5:d6,id=idd5QZro,vectors=4,netdev=id3eaS82,bus=pci.0,addr=0x4  \
    -netdev tap,id=id3eaS82,vhost=on \
    -m 8192  \
    -smp 4,maxcpus=4,cores=2,threads=1,sockets=2  \
    -cpu 'Westmere',+kvm_pv_unhalt  \
    -vnc :0  \
    -rtc base=utc,clock=host,driftfix=slew  \
    -boot menu=on,strict=off,order=cdn,once=c \
    -enable-kvm  \
    -device sga \

Actual results:
  Boot failed with error msg:
   ********
   Booting from Hard Disk...
   Boot failed:not a bootable disk
   *************
    
Expected results:
  Guest can be booted successfully from img "/home/kvm_autotest_root/images/rhel75-64-virtio.qcow2" 


Additional Info:
  Not a regression bug.