Description of problem:
In seabios boot menu, only one scsi disk can be listed

Version-Release number of selected component (if applicable):
libvirt-1.3.2-1.el7.x86_64
qemu-kvm-rhev-2.5.0-2.el7.x86_64
kernel-3.10.0-353.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.start a guest with the following xml:
 <os>
    <type arch='x86_64' machine='pc-i440fx-rhel7.2.0'>hvm</type>
    <boot dev='hd'/>
    <bootmenu enable='yes' timeout='7000'/>
  </os>
...
 <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/test.img'/>
      <target dev='sda' bus='scsi'/>     --->scsi bus
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/test.qcow2'/>
      <target dev='sdb' bus='scsi'/>        --->scsi bus
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>
...

2.In the guest,press 'ESC' to look at the bios boot menu,when the guest is booting.


Actual results:
In the boot menu,only one disk is listed.

Expected results:
In the boot menu,both disks will be listed.


Additional info:
for ide/sata/virtio bus,all the disks can be listed