Add support for transporting backtraces

The xapi-storage/python/xapi.py library pickles up the python backtrace
using json, this code parses it and converts it to the s-expression used
internally by xapi.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Regenerate OASIS

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #11 from djs55/experimental-backtraces

Transport backtraces through the API