backup: follow symbolic links
backup: follow symbolic links

For a long time Qubes backup did not include symlinked files, which
apparently is quite common practice for users with multiple disks (for
example HDD + SSD). It is covered in documentation
(https://www.qubes-os.org/doc/secondary-storage/), but better solution
would be to simply include symlinked files.

Restore of such files would (of course) not preserve the symlinks -
normal files will be restored instead. But that's fine. If the user want
to move the data to another location, he/she can do that and restore the
symlink.

The only possible breakage from this change is having a copy (instead of
symlink) to a VM icon. But storing that symlink in a backup was broken
for some time (because of --xform usage) and it is handled during
restore, so not a real problem.

This doesn't cover all the problems with symlinked VM images - the other
one is qvm-block behaviour, which would treat such images as non-system
disks, so easily detachable (which would break VM operation). But that's
another story.

Fixes QubesOS/qubes-issues#1384

QubesOS/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin