First rename volume to backup revision, regardless of revisions_to_keep,
then rename -snap to current volume. And only then remove backup
revision (if exceed revisions_to_keep). This way even if commit
operation is interrupted, there is still a volume with the data.
This requires also adjusting few functions to actually fallback to most
recent backup revision if the current volume isn't found - create
_vid_current property for this purpose.
Also, use -snap volume for clone operation and commit it normally later.
This makes it safer to interrupt or even revert.
QubesOS/qubes-issues#2256
11c7b4bb51