hello,I 've deployed libvirt, and it is OK to use kvm.But when I use virtualbox,I can not attach a device to a domain.

It reports the error that can't get device from device.xml
here is my xml file for device(device.xml):
<disk type='block'>
        <driver name='phy'/>
                <source dev="/home/lb/iscsitest/vdisk.img"/>
        <target dev='sdb' bus='usb'/>
</disk>

and the xml file for the vm:

 

<domain type='vbox'>
  <name>winxp_3D</name>
  <uuid>d2542bf2-3d56-41fa-a048-4f9a7ed3cf4a</uuid>
  <memory>787456</memory>
  <currentMemory>787456</currentMemory>
  <vcpu>1</vcpu>
  <os>
    <type arch='x86_64'>hvm</type>
    <boot dev='fd'/>
    <boot dev='cdrom'/>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
  </features>
  <clock offset='localtime'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>destroy</on_reboot>
  <on_crash>destroy</on_crash>
  <devices>
    <disk type='file' device='disk'>
      <source file='/root/VirtualBox VMs/winxp_3D/winxp_3D.vdi'/>
      <target dev='hda' bus='ide'/>
    </disk>
    <interface type='bridge'>
      <mac address='08:00:27:6f:c6:4f'/>
      <source bridge='tap0'/>
      <model type='Am79C973'/>
    </interface>
    <input type='mouse' bus='ps2'/>
    <graphics type='desktop' display=':0.0'/>
    <sound model='ac97'/>
    <video>
      <model type='vbox' vram='32768' heads='1'>
        <acceleration accel3d='yes' accel2d='yes'/>
      </model>
    </video>
  </devices>
</domain>