is_dirty() returned a false positive if the volume was merely the source
of a currently running volume. For example, if fedora-33:root was the
source volume for myappvm:root and myappvm was running - then is_dirty()
returned True for fedora-33:root, because fedora-33/root-cow.img
contains some allocated blocks (one 256 KiB chunk containing only the
header) in this scenario, even though fedora-33 is shut down.
FixesQubesOS/qubes-issues#6371