[libvirt] [PATCH] Log virRun command output, return with error

The attached patch adds some extra logging to the virRun command. The full argv is now logged with DEBUG, as well as the commands stdout and stderr (if anything is found). Also, if the command returns an error, we raise the stderr content with the reported error msg. This will significantly help with debugging certain issues, particularly with the storage driver which makes heavy use of virRun. Thanks, Cole