Multi-Version Drivers

[lindig]

MVD CP-52334 multi-version driver API/CLI

Provide an API/CLI for multi-version drivers. Two new classes represent
them: Host_driver and Driver_variant. See

  * doc/content/toolstack/features/MVD/index.md

for technical background and feature overview.

The current implementation mocks the missing drivertool that manipulates
the file system and observes driver state. We expect to integrate this
at a later point. Consequently, the current implementation is incomplete
and exists to faciliate development of API and XE clients.

On Xapi start and restart driver information is purged from the
database and re-built from scrach. This could be improved in the future
to be incremental.

[Vincent-lau]

I understand this is a draft version. But would be easier to split it into smaller commits for review, or is that not very feasible?

[lindig]

I worked from the original work by @last-genius but ended up having so many changes that the commit history was not meaningful. In addition, re-basing made a mess of them. In the end, this is difficult to split up - it's almost impossible to split this up such that each commit still compiles because it references each other. I could split it by module or something like it - but that is not entirely logical either.

[lindig]

I have additional code to make the database update incremental on scan() but would like to see the basics reviewed first.