Do not write directly to main volume, instead create temporary volume
and only commit it to the main one when operation is finished. This
solve multiple problems:
- import operation can be aborted, without data loss
- importing new data over existing volume will not leave traces of
previous content - especially when importing smaller volume to bigger
one
- import operation can be reverted - it create separate revision,
similar to start/stop
- easier to prevent qube from starting during import operation
- template still can be used when importing new version
QubesOS/qubes-issues#2256
2b80f0c044