In most cases it would be some leftover after failed restore, or even
the reason why the user is restoring a VM in the first place. Move it to
nearby directory, but do not remove - backup tool should _never_ remove
any data.
When the pre-existing directory would not be moved, restore utility
(`shutil.move`) would place the data inside of that directory, with
additional directory level (for example `/var/lib/qubes/appvms/work/work`),
which would be wrong and would later fail on `vm.verify_files`. And more
importantly - such VM would not work.
FixesQubesOS/qubes-issues#1386