for rbd devices, evaluate name field for graceful error message

When using rbd devices such as:

    <disk type='network' device='disk'>
      <driver name='qemu' type='raw'/>
      <auth username='libvirt'>
        <secret type='ceph' uuid='dc28392e-a277-4077-80ed-07040f5daffb'/>
      </auth>
      <source protocol='rbd' name='data/controller'>
        <host name='compute01' port='6789'/>
        <host name='192.168.1.2' port='6789'/>
      </source>
      <target dev='vdz' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>

Viewing the instance configuration causes a 500, as disk.xpathEval('source/@file|source/@dev')[0] throws an index error as one might expect. If we use the name field to lookup the storage volume, the instance view is displayed, however a graceful error is displayed:

https://drive.google.com/file/d/0B9DbsE2BbZ7uQzlrSEZBbG1UUnM/edit?usp=sharing

Signed-off-by: Tyler Baker <tyler.baker@linaro.org>