Use default VBD backend for empty CD drives

Now that the VDI is queried for a VBD backend at VBD creation, we need to
handle the case when the VBD is created with VDI ref NULL. This will be the
case for creating an empty CD drive, for example, on VM.import.

Since the model is to not add anything to the other_config field for default
behaviour then now we widen the exception handling such that _any_ failure to
find a specified backend kind results in the default.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>