backtraces: extend the design to include python -> OCaml

In particular this allows backtraces from SMAPIv1 services.

Signed-off-by: David Scott <dave.scott@citrix.com>
backtraces: restructure the document to make it more end to end

Signed-off-by: David Scott <dave.scott@citrix.com>
Move backtraces to the top-level features section

This has become a cross-cutting feature since it involves xapi, xenopsd,
the CLI and the storage backends.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #2 from djs55/python-backtraces

backtraces: extend the design to include python -> OCaml