libxl/gentypes.py: special-case KeyedUnion map handle generation

libxl/gentypes.py: special-case KeyedUnion map handle generation

Generate JSON map handle according to KeyedUnion discriminator.

The original JSON output for a keyed union is like:
 {
   ...
   "u" : { FIELDS }
   ...
 }

The discriminator is not generated, so that the parser won't be able to
figure out the fields in the incoming stream.

So we need to change this to something more sensible. For example, for
keyed union libxl_domain_type, which has a discriminator called "type",
we generate following for HVM guest:
 {
   ...
   "type.hvm" : { HVM FIELDS }
   ...
 }

Parser then can know the type of this union and how to interpret the
incoming stream.

Note that we change the existing API here. However the original output is
quite broken anyway, we cannot make sensible use of it and I doubt that
there's existing user of existing API. So we are acutally fixing a
problem.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>