Restore old code for calculating subdir within the archive. The new one
had two problems:
- set '/' for empty input subdir - which caused qubes.xml.000 to be
named '/qubes.xml.000' (and then converted to '../../qubes.xml.000');
among other things, this results in the wrong path used for encryption
passphrase
- resolved symlinks, which breaks calculating path for any symlinks
within VM's directory (symlinks there should be treated as normal files
to be sure that actual content is included in the backup)
This partially reverts 4e49b951ce.
FixesQubesOS/qubes-issues#4493
f023b3dd6e