rbd: ensure dev_file and dev_bus are never referenced before they are se...

When using rbd with qemu: elm.tag('file') and elm('dev') will not return true. So the variables dev_file and dev_bus will be referenced before they are assigned. This results is the instance graphing view stops functioning.

    <disk type='network' device='disk'>
      <driver name='qemu' type='raw'/>
      <auth username='libvirt'>
        <secret type='ceph' usage='client.libvirt secret'/>
      </auth>
      <source protocol='rbd' name='data/storage'>
        <host name='compute01' port='6789'/>
        <host name='192.168.1.2' port='6789'/>
      </source>
      <target dev='vdz' bus='virtio'/>
      <alias name='virtio-disk25'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>

This patch will initialize the variables and check that dev_file and dev_bus are set to a value other than None before they are used.

This fixes this issue:

https://github.com/retspen/webvirtmgr/issues/281