This driver isn't used in default Qubes 4.0 installation, but if we do
have it, let it follow defined API and its own documentation. And also
explicitly reject not supported operations:
- support only revisions_to_keep<=1, but do not support revert() anyway
(implemented version were wrong on so many levels...)
- use 'save_on_stop'/'snap_on_start' properties directly instead of
obsolete volume types
- don't call sudo - qubesd is running as root
- consistently use path, path_cow, path_source, path_source_cow
Also, add tests for BlockDevice instance returned by
FileVolume.block_device().
QubesOS/qubes-issues#2256
317d140f46