docs: Update upgrade-0.4 guide
conf: Add commented out 'lock_dir' option
archipelago: Propagate blktap ENOENT errno as VlmcTapdiskException

When trying to perform a vlmc command (e.g. showmapped) when
blktap-archipelago-utils is not installed, an OSError with ENOENT is raised.
Convert it to a Vlmctapdiskexception for better handling.