In POST:
http://post-office.corp.redhat.com/archives/rhvirt-patches/2011-October/msg00643.html

Verified this bug with:
libvirt-0.8.2-23.el5

Test steps:
1. Add the following disk xml info into guest(named "test") config file
# virsh edit test 
...
<disk type='block' device='disk'>
<driver name='qemu' cache='none'/>
<source dev='/var/lib/libvirt/images/disk-a.img'/>
<target dev='vdb' bus='virtio'/>
</disk>
<disk type='block' device='disk'>
<driver name='qemu' cache='none'/>
<source dev='/var/lib/libvirt/images/disk-b.img'/>
<target dev='vdc' bus='virtio'/>
</disk>
<disk type='block' device='disk'>
<driver name='qemu' cache='none'/>
<source dev='/var/lib/libvirt/images/disk-c.img'/>
<target dev='vde' bus='virtio'/>
</disk>
<disk type='block' device='disk'>
<driver name='qemu' cache='none'/>
<source dev='/var/lib/libvirt/images/disk-d.img'/>
<target dev='vdd' bus='virtio'/>
</disk>
2. start the guest
# virsh start test
3. # virsh dumpxml test
...
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/var/lib/libvirt/images/disk-a.img'/>
      <target dev='vdb' bus='virtio'/>
      <alias name='virtio1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/var/lib/libvirt/images/disk-b.img'/>
      <target dev='vdc' bus='virtio'/>
      <alias name='virtio2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/var/lib/libvirt/images/disk-d.img'/>
      <target dev='vdd' bus='virtio'/>
      <alias name='virtio3'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <source dev='/var/lib/libvirt/images/disk-c.img'/>
      <target dev='vde' bus='virtio'/>
      <alias name='virtio4'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
    </disk>
...
4. # ps axu|grep test
...
root     19558 75.9  4.3 2650128 355128 ?      Sl   16:11   0:40 /usr/libexec/qemu-kvm -S -M rhel5.4.0 -m 2048 -smp 2,sockets=2,cores=1,threads=1 -name test -uuid 55a5f876-b911-0f60-5275-897cb07cdd51 -monitor unix:/var/lib/libvirt/qemu/test.monitor,server,nowait -no-kvm-pit-reinjection -boot c -drive file=/var/lib/libvirt/images/test-1.img,if=virtio,boot=on,format=raw,cache=none -drive file=/var/lib/libvirt/images/disk-a.img,if=virtio,format=raw,cache=none -drive file=/var/lib/libvirt/images/disk-b.img,if=virtio,format=raw,cache=none -drive file=/var/lib/libvirt/images/disk-d.img,if=virtio,format=raw,cache=none -drive file=/var/lib/libvirt/images/disk-c.img,if=virtio,format=raw,cache=none -net nic,macaddr=54:52:00:69:43:54,vlan=0,model=virtio -net tap,fd=19,vlan=0 -serial pty -parallel none -usb -usbdevice tablet -vnc 127.0.0.1:0 -k en-us -vga cirrus -balloon virtio
...

After adding disks to guest, disks are reordered from vdb to vde. So move bug to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2012-0248.html