It is senseless to have full file path in multiple locations:
- external archive
- qubes.xml
- internal archive
Also it is more logical to have only "private.img" file in archive
placed in "appvms/untrusted/private.img.000". Although this is rather
cosmetic change for VMs data, it is required to backup arbitrary
directory, like dom0 user home.
Also use os.path.* instead of manual string operations (split,
partition). It is more foolproof.