While I have not as yet done this to the file the kvm VM machine is a file and you can move it and tar.gz ie its just 1s and 0s, so I cant see why bacula would have a problem. However I do presently backup the VM machines with bacula-fd and the director and sd are all Vms and I have restored files with no issues (both Vmware and kvm).
Just remember like a database the "file" will be constantly changing so the machine will have to be turned off when you do a backup to make sure it is consistent, hence I run a fd on each kvm which makes more sense IMHO.
If you do backup the one file you will have to restore the complete machine, boot it and extract the file(s) you want, hard work IMHO when having a fd on the client instead makes a restore trivial and quick.
