When using the xml as below:
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<devices>
  <emulator>/home/soulxu/data/work-code/qemu-kvm/x86_64-softmmu/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
    <driver name='qemu' type='qcow2'/>
    <source file='/home/soulxu/data/VM/images/linux.img'/>
    <target dev='vda' bus='virtio'/>
    <address type='drive' controller='0' bus='0' unit='0'/>
  </disk>
  <input type='mouse' bus='ps2'/>
  <graphics type='vnc' port='-1' autoport='yes'/>
  <video>
    <model type='cirrus' vram='9216' heads='1'/>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
  </video>
  <memballoon model='virtio'>
    <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
  </memballoon>
</devices>
------------------------------------------------------

Then can't startup qemu, the error message as below:
virsh # start test-vm
error: Failed to start domain test-vm
error: internal error process exited while connecting to monitor: qemu-system-x86_64: -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3: PCI: slot 3 function 0 not available for virtio-balloon-pci, in use by virtio-blk-pci
qemu-system-x86_64: -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3: Device 'virtio-balloon-pci' could not be initialized

So adding check for bus type and address type. Only the address of pci type support by virtio bus.
Signed-off-by: NXu He Jie <xuhj@linux.vnet.ibm.com>