libvirt: Handle `listDevices` unsupported exception

It's possible that `listDevices` will return an exception if it's not
currently supported.

If this happens, the compute service will fail to start up.

The proposed solution is to catch this error, log a warning, and allow
the compute-service to continue initialization.

Change-Id: If152abd1080e9ba395de5d8ea10f5e0c476d4e2b
Closes-Bug: #1304721
Merge "libvirt: Handle `listDevices` unsupported exception"