Wtf?

This is a fairly standard commit, with a standard commit message.
Update doc according to the compil changes

This doc update covers the first part of the change for the new way
dislocker is being compiled.
Change to way VMK are gathered

Accesses get the VMK, so put the code to get the VMK in these units. The
metadata/vmk unit still has some helper functions though.
Improve an error case of dislocker-metadata