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

core/backup.py

@@ -555,6 +555,7 @@ def backup_do(base_backup_dir, files_to_backup, passphrase,
         tar_cmdline = ["tar", "-Pc", '--sparse',
                        "-f", backup_pipe,
                        '-C', os.path.dirname(filename["path"]),
+                       '--dereference',
                        '--xform', 's:^%s:%s\\0:' % (
                            os.path.basename(filename["path"]),
                            filename["subdir"]),